Not much is known about Soubi's background. He lost both of his parents when he was six years old, and enrolled in Shichisei Gakuen (Seven Moons Academy), a school for Fighters, some time after. In the Seven Moons Academy, he became the student of Ritsu Minami,

Soubi is later partnered with Seimei, who requested Soubi specifically and whom Ritsu chose specially as a Sacrifice for Soubi. Seimei carved his true name, "Beloved", into the skin of Soubi's neck and told Soubi that it marked him as Seimei's, telling Soubi that such things as "love" are inconstant but that power and control last indefinitely. They became and are a well-known pair for their strength in battle, until Seimei was supposedly murdered by being set fire to in the classroom of Ritsuka Aoyagi, his younger brother's, elementary school.

Soubi's personality and his world have been shaped by primarily two people: Ritsu Minami and Seimei Aoyagi; later Ritsuka Aoyagi becomes a counterpoint to his earlier development.

Soubi's first interactions with Ritsuka, in particular, are full of miscommunications and tension, in part over Soubi's refusal to share certain information. He won't talk much about his past with Seimei, his former sacrifice and Ritsuka's brother, and he won't talk about Septimal Moon, the organization that ostensibly murdered Seimei. This refusal, however, comes in part from Soubi's desire not to confront the reality of his life before meeting Ritsuka.

Though Soubi readily deflects or avoids revealing more complicated emotions, his weaknesses and conflicts do occasionally show through. For most of his life, Soubi was groomed to be an object: as Ritsu worked with Soubi, he thoroughly destroyed Soubi's sense of self. Ritsu repeats that it is Soubi's fate to be dominanted, to submit, to protect his sacrifice, and fight. To discipline Soubi and to build his tolerance to pain, Ritsu abused him regularly under the guise of helping him be the best. Isolated and completely in Ritsu's care, Soubi accepted this as a normal part of a relationship. While at Seven Moons, Ritsu became Soubi's entire world. Other adults that might have been aware of what Ritsu was doing did not intercede to save him.

"I don't love him or hate him. I just have no one else."

Soubi has deep abandonment issues. His parents died when he is six years old; Ritsu took him in, abused him, and began a sexual relationship with Soubi which gave him the impression that Ritsu intended to keep him - instead, Ritsu gives him to Seimei; Seimei dies and leaves Soubi to Ritsuka. Even when it's revealed that Seimei is alive, he waits to take Soubi back and threatens several times to abandon him again. When Youji and Natsuo, who had been living with him, disappear to return to Septimal Moon, Soubi confesses to Kio that he believes he is "easily thrown away."

Soubi has been conditioned to consider himself an object, a tool, whose only purpose is to be useful to and controlled by someone else. He struggles with this when Ritsuka refuses to behave the way Soubi expects him to, and he often falls back on what he believes is true:

"I only need one person. He governs me completely. That is my choice."

"I want to be governed, I don't want to think anymore."

"My body is a vessel which exists only to obey orders. My will, my soul, my heart... all these things dwell in the palm of Ritsuka's hand."

It takes time before Soubi is able to accept that Ritsuka is really nothing like Seimei, and trying to get him to behave like Seimei is useless. This revelation rocks Soubi's world: for most of his life he's had someone telling him what to do and how to live. It isn't until this point that we start to see Soubi's inner thoughts and monologues. Ritsuka's refusal to objectify him opens a path for Soubi to develop further as a person than he could have under Seimei. In this period, Soubi proves that he can be kind and considerate; he can also be needy, judgmental, and kind of a jerk to people whose opinions he doesn't value.

Soubi's ability to regulate his emotions is inconsistent at best. He can be moody, passive aggressive, and occasionally violent. He is perfectly happy to break Nisei's fingers when he realizes that Nisei has replaced him as Seimei's fighter. When he learns that Seimei is alive, he has a small breakdown and begs Ritsuka to take him and run away, "Otherwise, I don't know what's going to happen." Despite dedicating himself to Ritsuka and becoming fond of him outside of Seimei's mandate, Soubi knows that if Seimei is truly alive, he will not disobey his former sacrifice. If Seimei is alive, his word is law, and that could take Soubi away from Ritsuka. This should be what Soubi wants - Seimei is his original sacrifice - but it clearly isn't. Despite his deep ties to Seimei, Soubi has developed enough under Ritsuka that he manages to warn Ritsuka that Seimei lies, and he is clearly unhappy when Seimei takes him back, even if he doesn't disobey. When what Soubi wants clashes with his training, he tends to break down or freeze. This happens again after following an order from Seimei that allows Seimei to escape: for the rest of the night, Soubi simply repeats "I don't want to" any time someone tells him to do something, including Ritsuka. It's a belated attempted to regain agency: something he never had under Seimei or Ritsu.

Though Soubi has considerable tolerance for physical pain, he has almost no tolerance for emotional pain. Because of this, he generally avoids talking about or confronting his past. He will change the subject or outright lie to get away from talking about that part of his life. He even lies to Ritsuka about how he lost his ears (a sign of virginity).

Losing Seimei and gaining Ritsuka put Soubi in a position in which he has to stand more on his own. He has to make his own decisions, including about what to feel, and he can no longer depend on someone dealing out physical pain to distract him from emotional pain. And there is a lot of it. Throughout this process, Soubi awkwardly transitions to someone who has no idea how to live on his own to someone who is attempting to find a balance between what he believes he is and what he could be.

Soubi's morality generally dwells in a gray area: he has ideas of right and wrong, but those are generally eclipsed by whatever his sacrifice wants him to do. A handful of times we see him judging other fighters for not following the rules of battle. He engages with a Zero pair to save Ritsuka's teacher, mostly because he thinks that Ritsuka will be angry if he does nothing and finds out about it. However, he also lets those same Zero fighters - who injure him badly - live with him when they say they have nowhere else to go.

Soubi is a fighter, one half of a paired unit. He has the ability to engage in spell/word-based battles either with his sacrifice partner or without them (though it is more dangerous for him to do so without).