A Prime Field Device and member of the Mitsurugi Agency who wields the BlazBlue Alternative. Ciel was taken aboard the Dimensional Boundary Submersible Warship Fugaku by Kagami Hajou after their Possibility was erased. Together with Raabe, Ciel Dives into Phantom Fields controlled by Observers in the hopes of freeing them and gaining insight on how to restore the Possibility that was lost.
Ciel and Raabe arrived into a Phantom Field that was set in the Magister's City of Ishana prior to its involvement in the Dark War. Ciel found herself fighting against Another Dark Mai, and the two locked into battle. A person interrupted their fight, and Mai went to attack them - as this happened, the person subconsciously used Phenomenon Intervention. Mai was distracted so Ciel took the opportunity to escape with the person and run to a safe location. Ciel mentally connected with them and she learned that their name is Rei. Ciel promised to protect them after introducing herself. As Rei awoke, Ciel hurried them to their feet, hoping to get them out of harms way since monsters were flooding towards Ciel and Raabe as they were Contaminants within the Field. Kazuma Kval found the trio, recognizing Rei and asking them if they were okay. Surrounded, Ciel and Kazuma fought against the monsters until there were none left.
As the monsters evaporated, Ciel noticed that Rei was an Observer. Another Dark Mai leapt down near the trio, and Ciel hurried Rei behind her. As the trio had just been attacked by monsters, Ciel carefully approached Mai and asked her if she was an enemy. Mai did not take well to the question, instead introducing herself as a "villain" and threatening to stab Rei repeatedly. Ciel asked Rei that they don't Observe Another Dark Mai too closely, and that she will protect them from her. Mai threatened to kill Kazuma, Ciel, and Rei, so Ciel shifted to a combat stance and asked that Kazuma take Rei far from the current location. In that moment, however, Raabe used Phenomenon Intervention and moved the group away from Another Dark Mai and onto another section of the island.
In the new area, Ciel noticed that Raabe had deactivated so started to hit it to try and reactivate it. Kazuma believed that they had been teleported, but Ciel clarified that it was just Phenomenon Intervention. Kazuma continued to probe, asking who Ciel is and where she is from. Ciel clarified that she was not from the island, but from the Mitsurugi Agency, who were investigating Ishana and an anomaly within it. Kazuma, having also noticed how different the island was to what he remembered, agreed to help Ciel in discovering the cauldron within the island. The group then moved further into the island to begin their search. Ciel listened in on Kazuma's explanation to Rei about Ishana and the Mage's Guild, but listening in had caused her to ignore her surroundings. She interrupted the explanation as the group were surrounded by monsters which they fought off.
During the battle, Ciel and Rei summoned Units. Kazuma asked what they were, so Ciel explained: by using her ability to organize data and combining it with Rei's Observational powers, they are able to utilize Chrono Phantasma (Those Who Should Not Exist) to fight for them. In order to avoid the coming waves of monsters, the group headed towards north-east towards the island port where communications would be. When they arrived, there were no ships, no people, and a communication device which was not working for mysterious reasons. A scream pierced the silence and Ciel, failing to convince Rei to stay back and out of trouble, followed the source of the scream, trailing behind both Rei and Kazuma. After battling away more monsters, they found Trinity Glassfille. She introduced herself and explained that Ishana's barrier had disappeared, so she went to investigate, and that there was supposedly a secondary defence mechanism that causes Ishana to isolate itself in another dimension. Knowing that Another Dark Mai was currently stalking Ishana to find and kill the group, they decided to head towards the Great Library to find a way to deactivate the mechanism and get off the island.
Trinity suggested that the group go through the sewerage of the island in order to avoid monsters and Mai. As they ventured through the sewers, Ciel tried to explain to Kazuma and Trinity that she was there to save them, but she struggled to find the words before they were surrounded by more monsters. When they came out of the sewer, they arrived in a park, again surrounded by monsters which they fought off. As the battle ended, the group walked up past the park - Trinity revealed that she had been here earlier but had lost consciousness before reawakening in a forest. Ciel had questions, but kept them to herself. As the group made their way towards the Great Library, they went past the Grand Clock; both Ciel and Kazuma saw it pointing at 3pm, while Trinity could see pointing past midnight. Ciel's suspicions about Trinity being the Observer of the field were growing.
Inside the Great Library, Ciel was stunned by the sheer volume of books. In order for Trinity to gain information to help Ishana, they had to enter a forbidden section of the library, activating a security system which the group had to fight. Once the battle ended, Trinity admitted she was hesitant to let Ciel inside the area since she is an outsider, but Kazuma managed to persuade Trinity to allow her in. Once inside the restricted section, the group scrambled to find as much information as possible. Ciel then theorized that the group could escape by going to the cauldron, something that she knew existed in Ishana because of records she had previously read in the Mitsurugi Agency. Trinity continued to bring up Konoe A. Mercury, someone else that Ciel had records on. On Ciel's advice, the group headed towards the cauldron, which lay in a sacred area to the Ishanan population - the Ishana Cathedral.
As the group made their way to the Cathedral, Trinity voiced concerns over whether the cauldron actually existed. It was here that Ciel confessed that she was going to destroy the cauldron. Trinity and Kazuma became suspicious of her, but Rei voiced their faith in her, leading the two mages to realize that she has only helped them since they met. Rei's faith in Ciel made her happy, and the group headed further into the Cathedral after fighting off more monsters. When they came near the cauldron, both Ciel and Kazuma told Trinity that they had deduced she was the Observer, and that Ishana had become like this because of an interference with her Observation. The entire island was just a construction of her Observation, and that she needed to recognize the cauldron in order for it to activate. More monsters appeared and the group fought them off. They continued to fight down towards the cauldron, being blocked by monsters seemingly defending the cauldron from them.
When the group finally made it to the cauldron, they were met by Another Dark Mai, who was waiting for them. She gleefully greeted them, taunting Trinity and claiming that she had killed her earlier on, but due to the nature of Phantom Fields, Trinity was resurrected and her memories wiped. Ciel asked her why she was interfering this much within this Phantom Field, but Mai danced around the question, saying that she had noticed some interesting things, referring to either Ciel or Rei. In order to test their strength, Mai challenged them to battle; once the dust settled, she remarked that she had seen enough and disappeared through the cauldron. Trinity then turned on the group, unable to let the cauldron be destroyed and for her desires to slip through her fingers. Ciel, Kazuma, and the summoned units fought against her, eventually winning. Ciel then activated her BlazBlue Alternative to destroy the cauldron and free Trinity from the Phantom Field. Trinity, coming to her senses, thanked Ciel. Ciel bode Kazuma and Trinity farewell as the Field began to reform into a Possibility. Just as she was about to return to the Fugaku, Ciel took Rei with her.
After arriving back on the Fugaku, Ciel took Rei to a room where they could rest. The following day, she saw Kagami trying to convince Rei to join them on their mission to free the trapped Observers. She interrupted, asking her boss to not bribe them; with this, she took Rei back to their room and thanked them for helping her in Ishana, confessing that Dark Mai might've killed her if not for Rei and that she was not able to activate the BlazBlue Alternative without them.
The following day, Ciel, Rei, and Raabe agreed to dive into yet another Phantom Field, continuing their mission. Raabe had been fixed and was able to speak again. Both Ciel and Raabe talked about how to best identify the Observer within the Field, concluding that the Observer would always have a Drive and would always subconsciously avoid the cauldron. Ciel then left to do reconnaissance, discovering multiple monsters enhanced by seithr and a nearby village. She returned to Raabe and Rei and they fought against the monsters to clear the way.
When they finally entered the village, the group met the villagers. Ciel's words and mannerisms made them wary, but this awkward interaction was cut short as soldiers from the Novus Orbis Librarium surrounded them. The NOL mistook the group as members of a local resistance, and they fought against each other.  The NOL were repelled but a version of Tsubaki Yayoi appeared and rallied the soldiers together. With their morale restored, numbers increased, and Tsubaki at their side, the NOL was able to capture Ciel, Rei, and Raabe.  They were taken to a cell in the 17th Hierarchical City of Yamatsumi. Tsubaki began to interrogate them, but upon understanding that they were not familiar with even the NOL, she instead educated them on the organization as well as the concept of a Hierarchical City. Ciel explained that they were in a Phantom Field, but Raabe told her to stay quiet as it was unnecessary information for Tsubaki to hear. Eventually Tsubaki left.
Both Ciel and Raabe decided that it was best to aid the resistance, feeling that by doing so they would be able to identify the Observer of the Phantom Field easier. They were interrupted by a woman in another cell, who picked up on that the group were not from this world. A explosion rang out throughout the building and the resistance came to rescue the woman in the cell – Bullet. She introduced herself as Kuon Glamred Stroheim arrived, asking them to follow her out of the NOL branch. The NOL gave chase. Rei, Ciel, Raabe, Bullet, and Kuon stuck together to distract the NOL from other resistance members who were freed. They fought against a security golem, besting it. 
Once outside of Yamatsumi, Bullet offered the group the chance to join her and the resistance. They turned her down, and instead Bullet told them of a village nearby where they could rest. On the way there, Kuon confided in the group that she was a Contaminant, so Ciel and Raabe explained the situation to her and how she could return to her own world. Eventually they arrived at the village and were able to rest at an inn. The following morning, they listened to the villagers impressions of both the resistance and NOL before leaving for another village.  At the next village, they found it was under attack and that Bullet was defending it. They joined forces, destroying the monsters that had attacked the village. Both Ciel and Raabe had noticed that Bullet was a Drive user, and so kept their eyes on her while they went to the next village together. On the way there, the group destroyed a large number of monsters which were blocking the way. When they arrived at the village, it was apparent that Bullet was adored by the village folk; she explained that she was fighting for her own pride and would do anything to help the group and the resistance.
Bullet later gave a mission to Ciel, Rei, Raabe, and Kuon: infiltrate Yamatsumi to rescue a trapped resistance member. They agreed to take on the mission and infiltrated the city without incident. They soon found the agent and agreed to escort him out during the night through the sewer system. Raabe stayed behind to guard him while the others went out for lunch and to gather information. Hoodlums approached the group after their meal, but Kuon soundly defeated them using explosive magic. This, however, attracted NOL guards. The guards let the group go, rousing suspicion in the group. When the night approached, the group left for the sewers, but were caught by patrolling guards. With little choices left, the group fought their way through them.
The sewers were filled with demons, which the group were forced to fight through. After fighting through hoards of monsters, Raabe and Kuon discussed how unusual the NOL was acting. Despite being on high alert for resistance members, the entrance into Yamatsumi had been easy and the guards did not care much for Kuon's explosive fight earlier that day. They were interrupted by a sudden attack from Ripper, who Ciel and Raabe immediately identified as a Contaminant. Ripper managed to severely injure both the agent and Kuon so Ciel stood forward and pushed Rei. Ciel battled against Ripper, but the battle soon turned in his favor as he landed a shallow cut on her. Before the battle could continue, Bullet burst through the other side of the sewer with members of the resistance for reinforcements; Ripper fled in response.
Bullet escorted the group back to a village where they could recover. Ciel, Rei, and Raabe decided that they would be going to investigate the NOL within Yamatsumi. Kuon stopped them, telling them that she would be joining as she felt responsible for Ripper since they were from the same Possibility. They made their way to the higher levels of Yamatsumi to get a better understanding of the city, but they were almost immediately spotted by Tsubaki and the NOL guards surrounding her. The guards attacked the group, but Tsubaki interrupted them, dismissing the guards. Raabe offered to give her information about the resistance so long as they could talk in private. Tsubaki agreed.< ref>BlazBlue Alternative: Dark War, Chapter 1, 7 – 2</ref> They moved to a more private area, where Tsubaki asked them to be mediators between herself and the resistance. Jin Kisaragi would be arriving in Yamatsumi soon with reinforcements to continue building and managing the city, and they were going to destroy the resistance as well. The group agreed to help as mediators to avoid bloodshed. 
On their way back to the village, the group was attacked by Ripper. Ciel, Raabe, and Kuon tried to get him to answer questions on who had sent him to attack them, but he declined to answer and attacked them. The fight was interrupted by Tsubaki, which caused Ripper to flee. They told her that he had been killing resistance agents under NOL orders, meaning that negotiations were going to be difficult. Regardless, the group still agreed to act as mediators. They continued their way back to the village but found Bullet and other resistance members in the middle of a raid. They learned she was going to steal a mechanical shaft that was essential to the development of Yamatsumi. They extended the offer for negotiations that Tsubaki wanted, but Bullet declined, telling them that she would never join the NOL nor would she allow them to continue to do as they please. To prevent them from leaking the raid to Tsubaki, Bullet ordered that the resistance members apprehend the group, but they were powerless to do so.
Ciel called Tsubaki and alerted her to the raid. Tsubaki relayed that those parts had already arrived. and that Bullet had likely been given false information. The raid would instead be on a civilian ship which could lead to fatal casualties. Tsubaki reunited with Rei, Raabe, Ciel, and Kuon and also made her aware that Bullet had declined their peace offering. Before they could act to stop her, they were interrupted by Hibiki Kohaku. He told the group they were under arrest as they had broken from the jail they were confined to earlier. After a fight, Tsubaki ordered the group to leave and stop the resistance while she held off Hibiki.  They caught up to Bullet and helped her escape from the NOL after telling her and the resistance that they had been set up. When they returned to the village, the group found it was destroyed with scores of dead bodies. Ripper had destroyed it and killed as many people as he took pleasure in doing so. Bullet swore revenge on the NOL, knowing that this order must have come from Hibiki Kohaku. She rallied everyone together and ordered that they would destroy the mechanical shaft. Kuon, Ciel, Raabe, and Rei pleaded with her to reconsider, but these pleas fell on deaf ears. The group decided that they needed to see Tsubaki.
The following morning, Kuon, Raabe, Rei, and Ciel were surrounded by the resistance, weapons raised. The group struggled, hoping to break free, but this ended when Bullet arrived and took Rei hostage. Ciel was held back by Raabe, as she could not guarantee Rei's safety by fighting back against Bullet. Bullet and other members of the resistance left soon afterwards. With few guards left, the group was able to bust out and free Rei. As the group made their way towards Yamatsumi, Raabe called Tsubaki and made her aware of what Bullet was aiming to do, and also told her about the massacre that Ripper had enacted upon the village. Tsubaki told them that the shaft the resistance was targeting was essential to Yamatsumi's infrastructure. Without it, the city would collapse. They were able to reunite with her and a group of NOL soldiers loyal to her.
The group ventured deeper into the NOL branch, finding no guards protecting the perimeter. Tsubaki noted that the soldiers were likely being led by Hibiki and battling the resistance on a higher level. They accidentally activated a security golem but were able to destroy it before proceeding.  Ripper then appeared, confessing to the slaughter of the villagers. The group battled against him, managing to defeat him and forcing his retreat. Ciel and Raabe made the decision that they would have to push forward to find Hibiki and Bullet before it was too late. When they finally found them, the two were locked in battle while soldiers from the NOL and the resistance lay nearby. Tsubaki pleaded with them both, begging the violence to stop with a passionate speech that resonated with the nearby soldiers. They pleaded with Bullet to stop the violence but she refused and the Phantom Field began to rewind back to before the soldiers agreed with Tsubaki. Now they turned against her; Kuon, Ciel, Rei, and Raabe defeated these soldiers. 
Reinforcements came to back up Bullet and it was then that Ciel and Raabe knew that Bullet was the Observer of the Phantom Field. They were able to conclude that all she wanted to do was to fight forever, and that any attempts to stop it would always be prevented. Peace or victory were never options, only violence. Ciel was then able to confirm the presence of a cauldron and explained that they could destroy it to free the field. Hibiki sided with the group, but Bullet then began to attack, not letting them take away the only thing she had ever known – violence. She struck Ciel, noting that she was tougher than she looked. Monsters flooded in, generated by the Phantom Field to exterminate the group.  The Phantom Field then began to operate to continue the battle forever, something that Bullet could not even control. Hibiki, under the Observation of the Field, changed sides to fight alongside Bullet, and the two attacked with combined forces from the NOL and the resistance.
Bullet's Observation then focused on Tsubaki as it tried to make her hostile towards the group. Tsubaki tried fighting back, but it was a losing battle until the arrival of Jin Kisaragi. He demanded that Tsubaki move to the backline while he stood with Ciel, Rei, Raabe, and Kuon to fight against Bullet.  The group was able to defeat Bullet, who accepted her the result. Jin stayed behind to look after Hibiki as everyone else went down to the cauldron. Kuon and Bullet promised Ciel that they would meet again before she destroyed the cauldron and returned with Rei and Raabe aboard the Fugaku. When they returned, Kagami congratulated them on the job. Ciel had taken substantial damage during the fight against Bullet and went for repairs. Afterwards, she and Rei had tea together.
- BlazBlue Alternative: Dark War, Prologue, 1
- BlazBlue Alternative: Dark War, Prologue, 2
- BlazBlue Alternative: Dark War, Prologue, 3 - 1
- BlazBlue Alternative: Dark War, Prologue, 3 - 2
- BlazBlue Alternative: Dark War, Prologue, 3 - 4
- BlazBlue Alternative: Dark War, Prologue, 4 - 1
- BlazBlue Alternative: Dark War, Prologue, 4 - 3
- BlazBlue Alternative: Dark War, Prologue 5 - 1
- BlazBlue Alternative: Dark War, Prologue 5 - 2
- BlazBlue Alternative: Dark War, Prologue 5 - 3
- BlazBlue Alternative: Dark War, Prologue 6 - 1
- BlazBlue Alternative: Dark War, Prologue 6 - 2
- BlazBlue Alternative: Dark War, Prologue 7 - 1
- BlazBlue Alternative: Dark War, Prologue 7 - 2
- BlazBlue Alternative: Dark War, Prologue 7 - 3
- BlazBlue Alternative: Dark War, Chapter 1, 1 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 1 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 1 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 1 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 2 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 2 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 2 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 2 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 3 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 3 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 3 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 3 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 4 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 4 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 4 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 4 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 5 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 6 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 6 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 6 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 7 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 7 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 7 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 8 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 8 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 8 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 9 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 9 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 9 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 9 – 4
- BlazBlue Alternative: Dark War, Chapter 1, 11 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 11 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 11 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 12 – 1
- BlazBlue Alternative: Dark War, Chapter 1, 12 – 2
- BlazBlue Alternative: Dark War, Chapter 1, 12 – 3
- BlazBlue Alternative: Dark War, Chapter 1, 12 – 4