The original Sechs died as a baby shortly after Lord Stroheim's first wife died in childbirth. Lord Stroheim devoted himself entirely to his magical research to find a way to recreate his son and succeeded in creating an artificial version of him.
One night, during his time in school, Lord Stroheim told Sechs the truth of his origins. Lord Stroheim said he did it for Sechs but Sechs didn't believe him and said his father would be punished if anyone learned about it, but he wouldn't turn his father in. Sechs then said he didn't want to hear anymore and ran off. Outside, he spoke with the Phantom Voice, who said he told Sechs he was a “doll” and that Sechs was a lot calmer than he expected. Sechs said that it made perfect sense and he felt a lot better since he always felt like he was different from everyone else, citing being able to speak to the Phantom Voice as an example. The Phantom Voice asked Sechs what he planned to do from then on and Sechs said he would leave his home and stay in the dorms, never to return to that house. The Phantom Voice wondered if that was okay since Sechs was their previous son, but Sechs says that didn't mean anything to him anymore. Sechs then asked the Phantom Voice to help him with something he had been wanting to do. For a long time, he had been thinking of how to end sorrow and now that he no longer was constrained by the limits of being human he could see clearly. He called the Phantom Voice his only friend and the Phantom Voice said that wasn't a bad way to be thought of. Sechs told the Phantom Voice that to end sorrow they needed to get rid of the concept of both life and death. The Phantom Voice promised to help Sechs find a way to do this.
One day, Sechs was watching a young Kuon as she climbed a tree. Kuon asked if she could become one of the Ten Sages and Sechs said he was sure she would be able to someday. Kuon said she wanted to be one so she could protect Ishana with her brother. Sechs told her it was dangerous to be so high up but Kuon didn't listen and fell out of the tree. Sechs managed to catch her but got injured in the process. Sechs said he was fine as long as Kuon was safe and she started crying, but Sechs assured her he was fine. They then went home.
At the age of 18, Sechs was appointed as one of the Ten Sages, the youngest of any mage in history.
Code: Embryo, Lost: Memories
In Ishana, Sechs told the Phantom Voice that it was finally time. He had finally found where the Kusanagi was and thought it was fate that his father was the one who knew, but taking the Kusanagi was only the first step. The Phantom Voice told Sechs that if he took up the Kusanagi his soul would be bound to it and he'd never be able to return to the seithr, but Sechs said that reaching the truth was the sole purpose of his life. He was then approached by Acht, who said she had just heard that he would finally put his plan into motion. Sechs said it would all begin the next day. Acht asked Sechs if he was sure about this since the guardian of the Kusanagi was Lord Stroheim and told him that if he wanted she could take care of it for him but he said it wouldn't be a problem.
The next night, Sechs, Drei and Acht met to being their plan. Drei told Sechs that only he embodied the attributes of a true Ten Sage, living honorably and using their power to lead other towards the truth, and he wanted to follow Sechs to see his plan come to fruition. Sechs said that it was pointless of Drei to ask for permission if he had already made up his mind. Drei apologized, knowing that Sechs hated wasting time and said he would follow Sechs of his own free will, promising to tear apart anyone who would stand in their way. Acht called him pathetic and Drei asked why she chose to follow Sechs but she refused to answer. Drei said he would not accept if Acht did not take this seriously but Acht replied that she was determined, revealing that she voluntarily became a Union. This proved her determination to Drei, who said he needed to rethink his opinion of Acht. Acht asked if he wanted to become a Union as well but Drei said that since he wanted to see Sechs's world with his own eyes he couldn't afford to take risks so early on. Acht once again asked Sechs if he was sure and he said yes.
Sechs met with Lord Stroheim, taking an elevator with him. Lord Stroheim wondered how many years it had been and said Sechs had grown. Lord Stroheim told Sechs that he was putting Sechs in charge of guarding the Kusanagi; that it was a duty that had been passed down through the Stroheim family for generations. Normally Lord Stroheim wouldn't give it Sechs since he left the family but changed his mind because Sechs became exemplary even among the Ten Sages. Lord Stroheim admitted that he was a bit jealous of Sechs since he lacked the power to become one of the Ten Sages. Sechs asked Lord Stroheim if he desired power and he replied that if he had it he wouldn't have to live with so much regret and wouldn't have put Sechs through so much hardship. Once the elevator reach the bottom, Sechs said he'd give his father the possibility for power and activated his Drive. However, it failed, killing Lord Stroheim.
Outside, Sechs told Acht that his father was an unworthy vessel. Acht was shocked that Sechs used his Drive and Sechs said he used it because Lord Stroheim desired power. Drei arrived, telling them that preparations for their escape were complete and they would be heading to Shin Yokozaki City. 
The next day he watched the encounter between Akira Kamewari, Kuon and Akio Osafune through a mysterious crystal. He rejected Acht's suggestion to go and attempt to get the Grimoire immediately, saying that sudden change will only lead to destruction. 
One night on the docks, he found a drunken man and attempted to turn him into a Union with his Drive but it failed, killing the man. Sechs said he was an inadequate vessel.
While searching for the wards set up by the Ten Sages, Touya, Es and Kuon found Sechs by the docks. When Kuon demanded to know why he betrayed the Magic Guild Sechs said it was because it was all meaningless and that the world is incomplete. Sechs then asked Kuon what she desires, to which she replied that she just wants him to come back to Ishana with her, revealing to Touya and Es that Sechs was her older brother. Sechs told Kuon that she should come with him if she wanted to know why he changed and was searching for the Grimoire but Touya asked Sechs if he was a Union and the one turning others into Unions, which Sechs confirmed. Kuon asked if he was the one who killed their father but Sechs said “that person” wasn't fit to be a vessel and it had no more meaning than that. He asked Kuon what sins he had committed, claiming that there was no such thing as death, he was simply returning people to the seithr they came from. He rejected Kuon's claim that he had been corrupted by becoming a Union; saying he spoke with the crystal and attempted to show her the “truth” by activating his Drive, but Touya got in his way. Touya yelled at Sechs for treating his sister like that but Sechs merely found him interesting before leaving. 
The evening after Akira removed his crystal, Drei, Sechs and Acht approached Touya and Souichiro Unomaru, since Sechs wanted to talk to Touya. When Unomaru said he didn't like that he was being ignored, Sechs called him foolish. Unomaru asked Sechs why he abandoned his status as one of the Ten Sages and Sechs said that to reach “zero” is completion. Sechs then asked Unomaru what he desired most from the world and Unomaru said he simply wanted the prosperity of all mankind and would do anything to achieve that, even controlling a god if one existed. Sechs asked Touya the same question but Touya didn't answer, merely wanting to know why Sechs turned Akira into a Union. Sechs said it was to show Touya the truth and asked him what he wanted again. Touya said he wanted to protect people from being hurt by people like Sechs, but Sechs asked him what he'd do if his existence caused people harm. He told Touya ask himself why he exists before he, Drei and Acht disappeared. 
Once Touya awakened the power of the Origin of the Grimoire, Drei and Sechs appeared before Kuon and Mei Amanohokosaka. Sechs told them that they needed the power of the Black Beast to destroy the Embryo. Drei told them that Unomaru was planning on using the Embryo to control the minds of everyone in the world. He warned Kuon and Mei not to get in their way or they would return everything to seithr and that all that mattered was the future Sechs was trying to create. Sechs and Drei then disappeared. Sechs then appeared before Es and Hinata Himezuru. He 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.
Later, Kuon found Sechs and asked him what he intended to do with Touya. Sechs said he would obviously have him unleash the Grimoire. When Kuon said that doing so would result in countless deaths and he shouldn't need to go that far to stop Unomaru, Sechs said he desired total destruction. Kuon was horrified that he didn't care if people die but Sechs said that nothing truly dies, just returns to its original state. He said it's a truth he learned from speaking with his crystal and that while “he” no longer spoke to Sechs, they would eventually become one. Kuon asked if this meant he was no longer her brother and Sechs said the conversation was a waste of time. Kuon said that if he was no longer her brother she would use the Sealed Spear: Izayoi to take him in. Sechs said he didn't have time to deal with it and disappeared. 
The next night, Acht spoke with Sechs about Drei's death, although Sechs insisted that Drei did not actually die, merely returned to seithr. Acht said she didn't have much time left and was starting to lose her sanity. Acht thanked Sechs for letting her be able to hold it together for that long but Sechs said he only did what was necessary. Acht was just glad she was able to be of use to him. She claimed that she was never that skilled despite being one of the Ten Sages, but thanks to Sechs she was able to see the world outside of Ishana. Sechs asked if there was a meaning to this conversation so Acht decided to leave. Before leaving, Acht told Sechs she loved him under her breath. Sechs then said that all the conditions had been met and all that was left was the finale. 
Once Unomaru completely refined the Embryo, Sechs suddenly appeared and stabbed him with the Sword of Destruction: Kusanagi. Sechs said that Unomaru's time had come. Unomaru ordered the Es-N units to kill Sechs but Sechs effortlessly defeated all of them. As Sechs turned his attention to the Embryo, Unomaru started stumbling forward, saying he wouldn't lose to Sechs but fell into the Boundary. Es arrived and 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 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 the 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.
Once Touya woke up, Sechs told him that Es had returned to the seithr. Touya demanded to know why Sechs could kill people so easily and to bring everyone back but Sechs said not to worry since Touya would soon be returned to the seithr as well. Touya then picked up the Signati Gladio Crystalis: Murakumo and said “Code: Soul Eater. XBLAZE, ignite!” Activating the true power of the Origin of the Grimoire, Touya declared that he would stop Sechs and attacked him. Touya was able to hold his own against Sechs's attacks for a while but Sechs said Touya would not be able to scratch him with that level of power and began his attack anew, overwhelming Touya. Sechs told Touya that he had grown stronger each time the world had been remade and even with the Origin of the Grimoire, Touya alone couldn't win. Touya then heard Es's voice telling him to come to her.
Back in the first iteration of the world, Touya heard a Discover Call and followed it to Sechs in the Restricted Ward. Touya and Sechs charged at each other and Touya managed to shatter Sechs's crystal. This destroyed the crystal of the Sechs in the present as well and he fell into the Boundary. 
- XBlaze Lost: Memories, Additional Scenario: Sechs
- XBlaze Code: Embryo, Episode 1
- XBlaze Code: Embryo, Episode 2
- XBlaze Code: Embryo, Episode 3
- XBlaze Code: Embryo, Episode 5
- XBlaze Code: Embryo, Es & Hinata Route Episode 8
- XBlaze Code: Embryo, Es & Hinata Route Episode 9
- XBlaze Code: Embryo, Es Route Episode 10
- XBlaze Code: Embryo, Es Route Episode 12
|Other (C-series)||Edgar and Claire Vermillion ·Ellen Nelson · Four · Francis and Rosenne · Lao Jiu · Pakumen & Terumen · Three · Two|
|Remix Heart||Elina · Guardian ·|
|Variable Heart||Julian ·|
|Alternative: Dark War|
|Crossover Characters (BBTAG)|
|Persona 4 Arena/
|Under Night In-Birth|