5 years before the Embryo Incident, Souichiro Unomaru created Es. Seeing that her eyes were not blue when she awakened, Unomaru declared her a failure and prepared to scrap her. Suddenly, Es spoke to him, causing Unomaru to name her Es and decide to turn her into a member of Sleipnir.
Code: Embryo, Lost: Memories
Es first appeared in the Restricted Ward. On his way to his part-time job, Touya briefly saw her but assumed it's his imagination. That night, Es and Mei Amanohokosaka battled a Union whose Drive let him control objects from a distance. Mei warned Es not to charge in blindly but Es ignored her, getting injured in the process. However, Es claimed the damage was minimal and defeated the Union. Mei angrily demanded to know why Es put herself at risk, but Es said that her safety was irrelevant if it meant completing the mission. A frustrated Mei left and told Es to report to the Mitsurugi Agency. Es wondered what Mei was trying to tell her. Es contacted Unomaru and told him the mission was complete. Unomaru then told her he had data on a Union in the Restricted Ward and for her to go there immediately, so Es headed out. Arriving in the Restricted Ward, Es heard a sound and found Touya Kagari being chased by Goro Joizumi. When Goro attacked Touya, she appeared from out of nowhere to protect Touya, slashing at Goro with the Murakumo. She then continued to beat down the injured Goro without mercy. Touya tried to stop her by jumping in between the two, so Es knocked him out. Es wondered what made Touya act so recklessly. She subdued Goro and prepared to hand him over to a Mitsurugi Agency recovery team. Es then received a new mission from Unomaru to head to Touya's house.
Es went to the Himezuru residence and was greeted at the door by Hinata Himezuru, who couldn't contain her excitement about how cute she found Es. Es asked if Touya was there, but Hinata said he hadn't returned. Es said that she would wait outside for him, which scared Hinata, who claimed it was far too dangerous for a cute girl like Es to be staying outside that late, but Es said she could defeat anyone who attacked her. Hinata didn't understand her comment about being a soldier, but invited Es inside anyway.  When Touya returned home, Es was waiting for him. She explained that her mission was to monitor Touya and protect him. Touya begged both her and her boss, Souichiro Unomaru, for her to leave but they refused. On Unomaru's instructions she forcibly stripped Touya to do a full-body examination in search of a crystal. After taking a sample of his blood, Unomaru and Mei left Es in the Himezuru residence. That night, Es came into Touya's bed to monitor him, forcing him to sleep on the couch. Es explained that she would continue to observe him even in the bathroom and that she would follow any order Unomaru gave her, even if it was to sacrifice herself. She was unable to understand Touya's anger about this.
Es left the next morning, telling Hinata Himezuru her mission was complete. Back at the Mitsurugi Agency's headquarters, she met with Unomaru. Unomaru asked her if anything strange happened with Touya and Es said he seemed like a normal human. Unomaru gave Es the mission to continue to guard Touya and told her she would be transferring into Touya's school. Unomaru tried Es to act natural around Touya's friends but Es was confused by the term friends, which annoyed Unomaru. At Hakuo North Academy, she introduced herself as a new transfer student under the name Es Mitsurugi. She then clearly stated her mission to protect Touya, causing him much embarrassment.  Once homeroom ended, Akira came over and demanded to know what Touya's relationship with Es was. When Es said she was “to be in constant contact with Touya,” Akira misinterpreted her words, making him furious. Touya could only yell that it wasn't like that. Touya eventually managed to explain the situation to Akira, relieving him. Akira then briefly hit on Es but she was just confused by his introduction and him saying he hoped they could be friends. Touya, Hinata tried to explain the concept to her. After school, she met with Touya on the roof. She told him that if he decided not to help the Mitsurugi Agency she would leave the school and return to headquarters to continue her previous mission. Touya then said that he would cooperate and she said she'd inform her superiors. Unomaru then called Es to tell her she would be staying at the Himezuru residence. She was then approached by Hinata, who had received the same information from Yuki Himezuru. Hinata was happy, saying they'd be like family and they left to head home.
On the way home from school, Touya heard a Discover Call so Es followed him in the direction he heard it from. They found Akira Kamewari and Kuon Glamred Stroheim after Akio Osafune had fled but Kuon quickly departed. At the Himezuru residence, Touya was bothered by Es living with them. Suddenly, Yuki returned home and, when she saw Es, grabbed and hugged her. Es was confused about why Yuki was showing her affection. That night, she took a bath with Hinata. Having determined that Akio had most likely progressed to phase four, Es and Touya went out in order to find him.  Hearing Akio's Discover Call coming from the Restricted Ward, Touya and Es headed there. Once they arrived they were attacked by Es, with Es deflecting Akio's shots. As Es continued to block his shots, Akio decided to just throw a giant steel tower at them but Kuon destroyed it, giving Es the opportunity to knock out Akio and bind his hands. After Akio woke up Es began interrogating him. When Akio refused to answer she beat him, much to Touya's horror. Once Drei arrived, she suggests they leave if Kuon and Drei began fighting, despite knowing that Kuon would lose.
The next morning, Es contacted Unomaru to tell him about Touya collapsing and about Kuon wanting to guard Touya, which he agreed to. She then mentioned that Akio got away because of the Ten Sages, which shocked Unomaru. He then told her to continue monitoring Touya.  She found Touya in the hallway and explained about Kuon becoming his bodyguard as well. When Touya wanted to speak to Kuon alone, Es went off with Hinata. 
After lunch, Kuon called Touya and Es to the roof and told them about the Ten Sages and the Origin of the Grimoire. She attempted to search the Mitsurugi Agency's database for information on the Grimoire but it had all been deleted.
Despite Es's suggestion that he not go to his part-time job at Maha Raja due to potential danger, Touya insisted he had to go. Es agreed but said she would wait outside the entire time. They then saw police cars going by but didn't do anything since Touya hadn't detected a Discover Call. While waiting outside, the manager offered her a seat inside. Ringo Akagi wondered about her relationship with Touya and asked if she was his girlfriend but she had no idea what that meant.
After work, Touya and Es saw Mei in the area where they saw police activity and went over to her. Mei allowed them to tag along with her as long as they didn't get in her way. After finding the night watchman's body, Touya sensed a Union and warned Es and Mei. Es deflected a thrown knife, but Ripper threw a smoke bomb at them. Es deflects a thrown knife but Ripper throws a smoke bomb at them. Ripper appeared behind Touya, but Es drove him off. Es pursued him while Mei guarded Touya. She followed Ripper into a building but he threw a Molotov Cocktail at her. Before Ripper could attack, Avenge arrived. After Ripper fled and Touya got knocked out by Avenge, Mei told Es to bring him home, to which she obliged. Once Touya woke up, Es apologized for allowing Touya to get hurt but Touya said it was his fault. Es asked him why he kept putting himself in danger and Touya explained that he couldn't stand seeing people get hurt and acted without thinking. He then requested that Es try to hold back from hurting people but she said she can't promise that since it might interfere with her mission. When they got home, they were greeted by Hinata, who took Es to take a bath with her. 
On the school roof during lunch, Touya told Es that he hadn't heard a Discover Call in a while and Es explained that it was because they had been finding many Unions who had been killed by Ripper. Hinata joined them and Touya asked Es where her name came from, but she didn't know.
At the Himezuru residence, Es told Touya about a series of Unions who had been found dead due to a Gainart episode. She deduced that this was being caused by someone trying to turn people into Unions before Kuon revealed it was one of the Ten Sages and asked her and Touya to help her find them, which she agreed to. At the location where Akira and his thugs attacked Akira, Kuon cast a spell on Es that would let her detect magic so she could help find the wards place around the city by the Ten Sages. After Kuon told Touya about being given permission to use the Sealed Spear: Izayoi, Es looked into the Mitsurugi Agency's database and learned about its cost. After Touya received a text from Hinata that she bought pudding, Es said they must finish their mission as soon as possible. Finding the last ward at the docks, the three encountered Sechs. After Sechs left Es confirmed that his crystal was still in phase zero, which should be impossible after using his Drive. Kuon asked for some time alone so Es left with Touya. 
Touya and Es arrived home to see Elise von Klagen staring at the Himezuru residence. Es told Touya about the ward around the house that should make it invisible to unauthorized people so anyone else who could see it was suspicious. Elise then introduced herself to them, explaining that she was sent by the Magic Guild and thanking Touya for taking care of Kuon.
The next day, Es followed along while Touya, Kuon and Elise went shopping. Touya and Es had a discussion about her thinking for herself, but Es simply said she couldn't ignore her orders. After shopping, they ran into Hinata and Akira. Upon realizing Akira's intentions, Kuon and Elise had Es help drag Touya away. At a café, they spotted Acht. When Acht prepared to attack with Ice magic, Es told Touya to stay behind her, ignoring his protests to protect Hinata instead.
The next morning, Es decided to call Unomaru and ask for maintenance due to having pain in her chest when she thought of Touya. After giving her report, Unomaru said he was too busy for maintenance and told Es it was an endurance test, an order she must follow even if it killed her and that she no longer needed to contact him. Later on the school roof, she, Touya, Hinata and Kuon were discussing Akira's absence that day. When Hinata left and Mei arrived, she and Kuon got into a fight. Touya said they seemed to get along well, which angered the two and confused Es. Due to her lack of maintenance, Es stumbled and figured it was likely she wouldn't survive the upcoming fight. They arrived home to find Acht holding an unconscious Hinata. Es told Touya that Acht was too powerful for her to defeat so they should give up on Hinata and escape, something that made Touya furious. He ran in to save Hinata himself but Acht simply blasted him away with Ice magic. Acht prepared to finish him off, so Es ran in to attack, prepared to kill Hinata to save Touya. However, she was restrained by Akira's Drive and got knocked out once Acht left. Es woke up to Mei treating her with special talismans, saying that they should help a little but she needs her regular maintenance. Mei was furious to find out that Unomaru intentionally was putting Es through this. After Mei left her to rest, Es started thinking about what happened. She could not understand why she was in pain for following her mission, her reason for living. Es then got up, wanting to speak to Touya, who demanded to know if she was seriously planning to hurt Hinata. When Es didn't answer, he walked away, saying he couldn't stand looking at her right now, with Es begging him not to leave.
The next morning, Hinata woke up fine, having been asleep through the entire event. She told Touya that Es was worried about him and had stayed outside his room the entire night but he quickly left. Yuki found it strange that Touya was actually mad and asked Es if they had a fight. Seeing Es looking sad, Yuki realized she wanted to make up with Touya, but Es didn't know what that meant. Yuki explained that it meant she wanted Touya to like her again and Es asked how to do that, but Yuki said Es would have to figure it out on her own, calling Es an adorable little sister. Es said she was not part of their family, but Hinata said she still thought of Es as a little sister regardless, which Es did not understand, however, Es wanted to be part of their family, but Hinata and Yuki said she already was an important part of their lives. Es was horrified that she nearly made a terrible mistake and didn't want to lose her family so Hinata told her she wasn't going anywhere. Yuki told Es to tell Touya the same thing if she wanted him to like her again and Es thanked her and left.  Later, she told Touya that she would take a three-hour break and that she informed Mei about this. She later found Touya and Mei at Maha Raja and took him back home. Once they get home, Kuon said she needed to talk to Touya and Es; telling them about Akira's unnatural crystal progression and that they needed to stop him before he reached phase five and was marked for termination. She said that nobody had survived being Union-infected and Touya should prepare for the worst. After receiving a phone call from Akira telling him to come to the school alone, Touya told Es that he heard a Discover Call coming from the abandoned factory. When Es pointed out that it was more than 500 meters away, outside the range of Touya's ability, he told her it's an order, so she went.
Outside, Es thought about how low she had fallen to ignore and order and wondered what to do. Remembering Hinata and Yuki's words, Es realized she wanted to be an important person to Touya. She then saw Touya leaving the house and decided to follow him. Es arrived on the roof of the school to stop Akira from killing Touya. She said she followed him despite being required to follow orders and wasn't sure why she did this. Es was unable to stand up to Akira's attacks and asked Touya to run away while she distracted Akira, even saying “please.” Touya angrily asked if she was planning on having him abandon her as well, but she said it's okay since she wasn't part of his family. When Es and Akira went to attack each other, Touya jumped in the way of their attacks, saying he didn't want them to kill each other. Touya reminded Akira that he used to jump into Akira's fights like he just did and that Akira promised he wouldn't fight anymore, bringing Akira back to his senses. Akira then ripped the crystal out of his arm. After collapsing, Akira apologized for breaking his promise before falling unconscious.  After Akira was taken to Shin Yokozaki University Hospital, Unomaru finally contacted Es and told her to return to the Agency so she said goodbye to Touya. However, right before she left, she told him to wait by grabbing his sleeve. Es apologized to him for what she nearly did to Hinata. Touya apologized as well and Es asked if he liked her again. Touya freaked out until Es explained that Yuki told her to make up with him. Touya said he wanted to make up with her as well and Es became very happy. Es then left.
Es went to the Mitsurugi Agency and received maintenance from Unomaru. While doing so, Unomaru said how excited he was to have such a rare sample as Akira's crystal. Unomaru asked Es about the talisman on her body and wondered whether Mei actually cared for a "worthless doll" if she went out of her way to do that. Once the maintenance finished, Unomaru left, saying he had important things to do and didn't have time to play with dolls. Es started to wonder if she really was a doll and if dolls could have feelings for people. She was then found by Mei, who told her that Hinata had been kidnapped by Ripper. In the car, the two discussed what they knew about Ripper. Mei found them on a roof in the Restricted Ward and they headed over. They saw Hinata falling and Mei used her magic to help Es catch Hinata. Showing her to Touya caused him to calm down after releasing the Origin of the Grimoire.  Mei told Es to take Hinata back home. On the way home she was found by Sechs, who looked at Hinata and said the Embryo's peaceful world would soon end now that the power of the Black Beast has been released. Es told Sechs to leave and that she would protect Touya and Hinata, but Sechs merely found it interesting that a "will" had manifested in a doll and asked her what she most desired. However, before she could answer, Sechs disappeared.
Back at the Himezuru residence, Es told Touya that Hinata was all right and Touya thanked her. Mei called for Touya to come down so he asked Es to keep an eye on Hinata for him, which she agreed to. Es watched over Hinata as she slept, saying she was glad she was alright. Suddenly, she heard Kuon shouting and ran out to find an angry Mei. After managing to calm Mei down, Mei explained that Kuon ran out to search for Sechs. Es then convinced Mei that Kuon actually did trust Mei, surprising her that Es was acting as the voice of reason. Mei then received a call from Unomaru, who had her tell Es to return to the Mitsurugi Agency. Mei warned Es about the Ten Sages saying Unomaru was planning to control the minds of all mankind and said Es would have to choose a side eventually.
Upon arriving at the Mitsurugi Agency, Unomaru told Es that he had found the Embryo and about its power to use Phenomenon Intervention, as well as the Boundary. As Es attempted to get more information, Unomaru remembered why he called her and gave her a new mission; to capture Touya, who was a dangerous Union. Es doubted Unomaru's claim and Unomaru threatened to dispose of her if she rejected the mission. Now that both the Ten Sages and the Mitsurugi Agency were after Touya, Es struggled with what to do but ultimately said yes to the mission.  Es found Touya at the docks but upon remembering Sechs's words that she had a will, she decided she wanted to be with Touya and approached him, telling him of her mission. Learning that Mei was in charge of the unit searching for him, Touya said he needed to find Kuon and asked Es to wait on capturing him until then, but Es said she wanted to guard him. Touya refused when she said she'd be killed for disobeying orders and said that she'd be at risk if he lost control again, saying she was too important to lose. Es promised that if Touya ever lost control of the Grimoire she would stop him no matter what so that he could not hurt his loved ones, which he agreed to. Before they could escape into the sewers, Mei found them. Touya told her about Es's promise to kill him and Es said she knew she would die for betraying the Mitsurugi Agency, which caused Mei to quit the Agency in frustration, saying the two need to care about their own lives more and consider how those close to them would feel if they died. Mei told Touya she would find Kuon so he should focus on escaping. Mei covered for them with a smoke bomb and they escaped underground. They were nearly caught by agents from the Mitsurugi Agency, but were rescued by Avenge, who took them to somewhere where they'd be safe. When Avenge told Touya about the Takamagahara Group and their attempt to create the T-System, Es explained about the Embryo's power to use Phenomenon Intervention and that with it it would be possible for the T-System to control the thoughts of everyone on the planet. Es said that the T-System would be impossible since they didn't have an energy source big enough for it, but Touya realized that they intend to use the Boundary and its infinite supply of seithr. Suddenly Es detected a large number of Mitsurugi Agency soldiers and Avenge told them to go ahead while he dealt with them.
After they left and discussed what to do next, they were attacked by Drei. Drei claimed he was aiming for Es, calling her a “doll.” Touya angrily retorted that Es was no different from a normal human but Drei said that artificial beings were not part of Sechs's plan and therefore could not exist.  Drei attacked and Es but Touya took the attack for himself. Drei told Touya to stand aside and watch him kill Es, attacking her again. As Touya started to lose control, he told Es to fulfil their promise, but Drei stopped Es from killing him. Touya unleashed the power of the Origin of the Grimoire on Drei and told Es to get behind him. Es asked if he could control the Grimoire and Touya said he didn't know but it felt like he could. When Touya refused to back down, Drei said that he could still break every bone in Touya's body, as long as he didn't kill him. Touya threatened to unleash the Grimoire again, but nothing happened. As Drei was about to attack he was stopped by Avenge. Avenge told them to leave Drei to him and run to Mei. Touya thanked Avenge and he and Es ran off.
After they escaped, Touya asked Es if Avenge would be all right and Es said it was possible since Drei was weakened. Es then asked Touya if he could control the Grimoire but Touya said he wasn't sure he had a hundred percent control since he couldn't use it now. Es collapsed, but Touya caught her. Touya apologized, saying she was probably cold because Touya's clothes were wet but Es said he's very warm. Emerging from the sewers, the two encountered Mei, who said she still hadn't found Kuon. Mei yelled at them for smelling and said she had a spare set of clothes for Es. Es thanked Mei, which embarrasses her. Touya, Es and Mei decided to return to the Himezuru residence and were greeted by Hinata. Es said that Hinata seemed to be herself and Hinata said it was a strange comment from Es. Es was confused about that but Hinata was just glad that she was in a good mood again. Mei asked Hinata if anything out of the ordinary happened, but Hinata said she'd been sleeping until recently. Mei then asked her if Kuon had returned and Hinata said she was upstairs. Hinata then realized she promised Kuon not to tell Mei about her return and begged Mei to pretend she didn't hear but Mei refused. Hinata was afraid that the two would start fighting but Es said it's okay because Mei and Kuon were friends, which took Mei aback. After everyone met downstairs, Es asked Hinata to get pudding for her, which she happily accepted. Mei asked if they had anything they needed to do before they headed out to confront Unomaru and Es asked for pudding so Mei begrudgingly said they'd wait until Hinata returned. Just as Mei complained about Hinata taking a long time to get back, she arrived with pudding. 
Arriving at the Restricted Ward, Mei found it odd that there was no one there. Es didn't detect any human presence inside and said if they were going to go inside, they should do it now. Inside, they were suddenly attacked by Acht's Ice magic. Es deflected all of the attacks and Acht remarked that she hoped to kill at least one of them with that attack. Acht said she had no idea where Sechs was and was no longer involved with him, she just wanted to go crazy. Acht said that they must defeat her in order to go on. She told Touya to go away since she didn't want to deal with him, but then said she was just kidding and attempts to kill him. However, Es protected him. Acht called Touya pathetic for always having others protect him. She then attacked him again but was intercepted by Mei, who told Touya to go home and he ran off. Kuon told Es to go with Touya and leave Acht to her and Mei. Kuon then created a large explosion that blinded Acht, allowing Es to get past her.
Upon reaching the Wadatsumi Research Facility, Touya was met by Es, who asked why he hadn't returned home. Touya asked Es why she was there and she said she figured Touya wouldn't listen to Mei based on his past actions. Touya said he meant what would happen to Kuon and Mei without her, but Es said she had confidence that they would be okay. Touya said he didn't want anything to happen to her, but Es said she had made a promise to him and Touya apologized for trying to break it. They then headed in together. 
Inside the Wadatsumi Research Facility, Touya and Es found it odd that they hadn't run into anyone yet. Es said that they had passed many security systems and none of them activated, suggesting this was a trap. Suddenly, they were confronted by an Es-N unit and Unomaru talked to them over the speaker system. Unomaru told Touya to go into an elevator and come down to him if he wanted to talk, but specified that only Touya was welcome. Unomaru called Es garbage and ranted about how much superior the Es-N units were. Unomaru told the Es-N to kill Es and capture Touya so Touya and Es ran. Es said that she would stay and fight Es-N while Touya went on ahead. Es told Touya that she had killed many Unions without questioning orders, but she had realized that what she did was wrong, saying she had committed so many sins she was beyond redemption, but hoped to try by protecting Touya. Touya went to confront Unomaru while Es stayed to fight Es-N. Es-N had all of Es's combat data, allowing it to predict her actions, making the battle difficult for Es. Es figured out a way around this by using unknown attack patterns and predicting Es-N's movements, allowing Es to defeat her. Es then killed the Es-N unit before moving on. 
Es reached the elevator to the basement but was ambushed by three more Es-N units. The three units quickly overwhelmed Es. Kuon and Mei then arrived to back Es up. Es told them that the Es-N units had their combat data and could predict their moves as well as that Touya went on ahead. Mei and Kuon said that was bad since the Grimoire was needed to refine the Embryo. They then told Es to go on ahead and leave the Es-N units to them.
Es arrived at the core of the T-System to find Sechs in front of Hinata. Es ran over to Touya, asking him if he's all right and what happened. Touya told Es that Hinata was the Embryo, that Sechs was going to kill her and that he had already killed Unomaru, asking Es to save Hinata. Es told Sechs to get away from Hinata, but Sechs called her a “doll” and told her if she got in his way he'd eliminate her as well, attacking her with the Sword of Destruction: Kusanagi. Es wondered how any human could have that much power and Sechs explained that it was because of the Kusanagi and Hinata's Phenomenon Interventions. Sechs said that destroying the Embryo would free the Boundary, flooding the world with seithr and melding everyone's souls into one. Sechs then fired the Kusanagi at Hinata, who fell into the Boundary. Touya tried to jump into the Boundary after Hinata, but Sechs said he would send Touya back to seithr and attacked him with Kusanagi. Es approached Touya to ask if he was okay so Sechs told her she was in the way and prepared to attack again, but Kuon and Mei arrived and attacked him with a combined spell, which he easily dodged. Sechs quickly defeated Kuon and Mei and turned his attention towards Es, disarming her with Kusanagi. Sechs found it interesting that Es feared death, calling her “the incomplete one.” Sechs then stabbed Es with Kusanagi, causing her to fall into the Boundary.
Es woke up in the Boundary and said she needed to focus on her sense of self or she'd be overwhelmed by the amount of seithr. She then heard Hinata's voice calling for her. Es realized that Hinata had lost her body and Hinata apologized, saying she wouldn't be able to make dinner anymore. Es said she could no longer protect Touya but Hinata responded that it's not true and she knew everything. Hinata told Es she could give Es her “heart” and if she did they might be able to save everyone. However, if they did Es might never be able to return. Es said that her only wish was to protect Touya so she would accept Hinata's heart. Hinata then gave the Embryo to Es and Hinata revealed that Es's real name was Embryo Storage. Hinata apologized, but Es said she made the choice of her own free will. Hinata then told Es she must touch the Master Unit and if she did she would understand everything. Hinata asked Es to save Touya. Es then heard a Discover Call coming from the depths of the Boundary. Following it, Es touched the Master Unit and saw visions of the world Sechs was trying to create. Using the Embryo's powers, Es pulled Touya into an enclosed space with her. Touya asked what happened to Hinata and Es reassured him that she'd restored Hinata's body and returned her to the Himezuru residence as well as that when Hinata woke up she would not remember that she was the Embryo. Es said that with Phenomenon Intervention she could return everything to when it was peaceful, but it would not work against Sechs because he had the Kusanagi. The Sword of Destruction: Kusanagi nullified Phenomenon Intervention and so Sechs retained his memories whenever the world was reset. Es told Touya that Hinata had already recreated the world many times without control of the Embryo. Hinata did this to try and protect everyone, but each time distortions would occur and Sechs would get stronger. Touya asked Es if there was anything he could do and Es responded that only he could do something about it. Since this phenomenon was born from an event in Touya's past, the current version of him should be able to change it. Es told Touya that she would send him to where everything began, the first iteration of the world, and once he was there he must eliminate a Union. Es apologized that she was forcing Touya to kill someone, but it was the only way. Touya understood that this was a sin he had to endure and accepted. Es told him he only had one chance to intervene and sent him to the past. She also used most of the Embryo's powers to create the World of the Tsukuyomi as a buffer dimension between Touya's world and the Boundary, sealing the Gate of Sheol. 
The guardian of the Azure Gate. Wielding the "Signati Gladio Crystalis: Murakumo," she tests the qualifications of those who have obtained the Azure. Even she does not know the answer to where she comes from, or why she tests their abilities, but she continues to act to fulfill her sense of duty to "guard the Gate." 
Act 3: The Replacement Blue
|Stage 3: Makoto Nanaya|
Es: This is... The present? It doesn't appear to be an illusion or a temporary space... Which can only make it, the Embryo? But why...?
Es: ...!? You're--!
Makoto: Huh? Me...? Uhh, have we met before?
Es: ...No. I don't believe I've met you, specifically. You just really resemble someone I know...
Makoto: Hmm... Like a doppelganger? Not really sure what that's supposed to mean but... A lot of people have been telling me that lately. Is my face really that common?
Es: However... The fact that I have encountered you here can only be the beckoning of the Azure Will.
Makoto: The Azure... Will...?
Es: Thus, I must confirm... If the will is truly calling for me. What it wants from me... Perhaps that is the key to understanding the reason I was brought to this foreign land...
Makoto: H-Huh!? Is THIS what you meant by confirming!? N-Now just wait a sec...! Yeah, you're not stopping, are you... Alright, in that case BRING IT ON!
Makoto: Damn, you're strong... What on earth are you!?
Es: I see your soul really does resemble someone's... It's almost, nostalgic.
Makoto: Is it? Soo... Were you able to confirm what you wanted? Something about the will or the key or whatever...
Es: ...I'm afraid I still do not know. Why did the Azure Will teleport me to this land...?
Makoto: This...land? Um, I'm not really sure what you mean. But if you wanna find the answer to something, I've found it's good to take a look with your own eyes.
Es: My own "eyes" you say...? Very well, I shall do exactly that. Moreover, I need to find a way to return to whence I came.
|Stage 6: Naoto Kurogane|
Es: ...Kurogane Naoto. A foreign entity from a foreign world... Are you part of the Azure Will, as well?
Naoto: Azure Will...? You know something about the Azure!?
Es: ...Nothing I care to share with you. Nonetheless, I am glad we met here... You are an irregularity. Perhaps you may be the clue for why I've been brought here.
Naoto: Don't you belong in this world? And that number above your head is insane...! Who the hell are you?
Es: The name is Es. I'm the gatekeeper and Arbiter to the realm of the Azure.
Naoto: The gatekeeper... To the realm of the Azure? I'd sure love to hear more about this.
Es: I have nothing more to say. What I do have for you... Are several questions.
Naoto: Heh, that's some nerve you've got buddy. Frankly, I really don't wanna fight a monster like you, but... ...I can't just back down from a guy with all the answers. I'll make sure you tell me everything I need to hear. Everything!
Naoto: Dammit... This isn't good. I had a bad feeling, but... You're in a completely different dimension. Like a damn mythical creature...
Es: I sense the power of the Azure from you. Its fragment...
Naoto: Heh, that explains why I feel like shit right now. I'm really not into fighting straight shooters like you. Sooooo if you'll excuse me!
Es: Wait...! Hm? This sensation... Chasing him is dangerous...? He has the means to... Defeat me? ...Very well. If it is indeed the Azure Will that brought me here, then there must be some significance to my meeting with him. Then our paths will no doubt cross once more. All shall be as the Azure commands...
|Stage 7: Izayoi|
Es: Cauldron... PFDs... Embryo Storage... Though the worlds in which they exist differ, there is no discrepancy among the people who live within... Which can only mean that...
Izayoi: You there, woman. Why do you trespass?
Es: I would never mistake that attire... You are Izayoi...
Izayoi: This sensation... is Noel? But there's something different... Something...
Es: Does this encounter have significance, too...?
Izayoi: What on earth are you...? This is no place for a mere human being.
Es: I'm searching... For the reason I am here. And what I must do. To seek what the Azure expects from me...
Izayoi: What you must do...? Who are you? Another Prime Field Device...? That doesn't seem quite right...
Es: Am I to conjecture that this too is the result of the Azure Will...? If that is the case, then I must see with my own eyes. What you are to this world... And what I am...
Izayoi: I see you're not bluffing... Then I'm afraid I cannot hold back, either!
Izayoi: K'uh... How dare you look away during a match!?
Es: I sense a presence... It's coming... The Azure...
Izayoi: SHOW YOURSELF! Ragna the Bloodedge! What are you doing here!?
Ragna: What the hell do you care? I've got business here. Simple as that... And who the hell are you!?
Es: ...I see. It all makes sense now. The reason I was brought to this world... My duty while I am here... Ragna the Bloodedge. I shall guide you to the gates. That is the will of the Azure.
|Stage 8: Ragna the Bloodedge|
Ragna: Wha...!? Where the hell is this? What do you want from me!?
Es: You are standing before the gates... The Gates to the Azure. I am one with the possibility of the Azure -- it is my duty to lead you to the gates. You can call me the Arbiter.
Ragna: The gates...? That's what this is!? So, what's the Arbiter want with me? You gonna let me do whatever I want with these gates? Somehow I doubt that's the case...
Es: By all means... Please, suit yourself.
Ragna: ...Huh? You're joking, right?
Es: However, that is only if you are indeed the Man of the Azure. It is also my duty to confirm that you are... That is the Azure Will.
Ragna: I don't care whose will it is... All I gotta do is kick your ass, right? Well, don't blame me for what's about to happen. Remember, you asked for this!
Es: K'uh... Ah--! ...I sense an Azure... Different from any other... And... A very strong will... A very strong desire... But... If you truly are the Man of the Azure... Then perhaps this too is the Azure Will... Then it chose him... And not me...?
Ragna: The hell are you mumbling about!? I'll finish you off--!
Es: Huh...? Ragna the Bloodedge... Disappeared...? Don't tell me that that Intervention...!
???: ...Too soon.
???: That man does indeed have the potential to become the Man of the Azure. But that possibility is still incomplete... He is not ready to open the gates.
Es: Incomplete... You can Observe that...? No... It's because you're you... You can Observe it.
???: The cycle of cause and destiny will continue as the Azure desires. Simple as that...
Es: That is the will of the gods?
???: ...There are no gods.
Es: ...Then I shall return to my duty of guarding the gates. (And once more lead him this way...) (...Should he be complete, and ready to accept his possibility. Then and only then shall the gates be...)
Summoned by the Azure, Es was made to guard the Gate to the Azure Horizon, becoming the Azure Guardian.  She stopped Mu-12 from passing through the Gate and battled her, defeating her and sending her back to the world she came from.  Another intruder arrived in the form of Naoto Kurogane – she assaulted him before throwing him back to the possibility he had just arrived from. 
In the final hours of Doomsday, Es ventured beyond the Gate to retrieve the data of the chosen who had lost their qualifications by accepting Doomsday and not pursuing the Azure. One of these visits included going to see Kokonoe, who she told of Doomsday was still commencing despite Izanami's defeat. She soon left to find the other chosen and back them up. 
Her next target was Makoto Nanaya, who strongly reminded her of Yuki. She backed up the demi-human's data and tried to eliminate Naoto, who was also there. She failed in doing the latter as Makoto blocked her path, but she was able to defeat her. Amane Nishiki arrived, seeing Makoto off as she disappeared into seithr, and telling Es that she could eliminate Naoto as he is outside of his jurisdiction. Yuuki Terumi, who had retaken the Susano'o Unit to become Susano'o. Amaterasu had already Observed that she would lose the fight, and without any possibility of winning, Es lost the power to use the Azure as well, being unable to activate Murakumo. She was nearly killed by Susano'o, but just managed to escape, teleporting herself next to Naoto.  She told him he was the future  before teleporting back to the Gate where she met Amane. They talked briefly until Es began dissolving into seithr.  She was last seen near the Gate before disappearing to an unknown location. 
- XBlaze Lost: Memories, Azure Main Memory 01
- XBlaze Code: Embryo, Episode 1
- XBlaze Code: Embryo, Episode 2
- XBlaze Lost: Memories, Azure Main Memory 02
- XBlaze Code: Embryo, Episode 3
- XBlaze Code: Embryo, Episode 4
- XBlaze Code: Embryo, Episode 5
- XBlaze Code: Embryo, Es & Hinata Route Episode 6
- XBlaze Lost: Memories, Azure Main Memory 03
- XBlaze Code: Embryo, Es & Hinata Route Episode 7
- XBlaze Code: Embryo, Es & Hinata Route Episode 8
- XBlaze Lost: Memories, Azure Main Memory 04
- XBlaze Code: Embryo, Es & Hinata Route Episode 9
- XBlaze Code: Embryo, Es Route Episode 10
- XBlaze Lost: Memories, Azure Main Memory 05
- XBlaze Code: Embryo, Es Route Episode 11
- XBlaze Code: Embryo, Es Route Episode 12
- XBlaze Lost: Memories, Azure Main Memory 07
- BlazBlue: Central Fiction Consumer Edition Official Site (JP), Story - Es
- BlazBlue: Central Fiction, Story Mode, Episode 005, Chapter 036, Sub Scenario 2
- BlazBlue: Central Fiction, Story Mode, Episode 006, Chapter 043, Sub Scenario 1
- BlazBlue: Central Fiction, Story Mode, Episode 007, Chapter 053
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 092
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 093
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 093
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 094
- BlazBlue: Central Fiction, Story Mode, Episode 011, Chapter 094, Sub Scenario 1
- BlazBlue: Central Fiction, Story Mode, Episode 012, Chapter 100