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, who was said to have been the Sacrifice of Soubi's mother (which was why they met).
Ritsu became a very influential figure in Soubi's life and it is Ritsu's strict teaching that raised Soubi to become the well-known fighter he is, but despite being teacher and student, their relationship was closer to that of a master and his slave. It is also mentioned that Ritsu is responsible for Soubi's fear of graves and the dark as a child, having brought Soubi as a child before the grave of his parents and telling him to get into the grave as well.
Ritsu was also responsible for getting rid of his ears at a very young age, explaining to Soubi that he did it to make him a better fighter. Whether it was a consensual act or not was not specified on Soubi's part, but it was hinted by another teacher at the school that Ritsu had desired Soubi because he had a face similar to his mother's.
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 shows up to greet Ritsuka Aoyagi after school and defends him from the Breathless pair that have come to abduct Ritsuka, introducing Ritsuka to the world of Spell battles. Soubi next meets Ritsuka to tell him to check Seimei's computer for a will, whereupon Ritsuka finds out that Soubi used to be Seimei's Fighter. Soubi reappears later to fend off a second abduction attempt by Breathless.
Soubi shows up at Ritsuka's house where he causes Hitomi Shinonome, who was there on a home visit, to cry and chases her off. He takes Ritsuka to battle and defeat Sleepless, allowing Ritsuka to obtain a coded missive from them on how to meet Septimal Moon. Soubi and Ritsuka return to Ritsuka's bedroom where Ritsuka treats Soubi's wounds.
Soubi intervenes when the male Zero pair try to rape Hitomi Shinonome and defeats them single-handedly, refusing to call on Ritsuka, and becoming injured in the process. Soubi takes home the defeated Zero pair and allows them to live there. Soubi wishes to call in on Ritsuka but leaves suddenly without giving a reason, and Ritsuka only realises that Soubi was injured after he leaves. Hitomi explains the situation the next day and Ritsuka e-mails Soubi the next day to order him to never again lie to him or to withhold information.
When Ritsuka solves the riddle of the coded message and goes to meet Septimal Moon, Soubi tries to join him but is too late, so he goes to Ritsuka's house and puts Ritsuka to bed.
The female Zero pair show up at Ritsuka's school to meet and fight Soubi and Ritsuka, and Soubi leads them away from the school to fight them himself without Ritsuka. He lets himself be defeated after the female Zeroes tell Soubi how hated he is by Nagisa, and Soubi thinks that he is “used to being hated” and submits himself to a vicious beating.
Soubi calls the male Zero pair for aid, but is found by Ritsuka accidentally, and the male Zeroes and Ritsuka end up conveying the injured Soubi home in a taxi. The female Zeroes appear at Soubi's apartment and Soubi sends the male Zeroes down to fight them. The male Zeroes are defeated and Ritsuka agrees to fight the female Zeroes later.
Soubi phones his old teacher Ritsu for information on the female Zeroes and Ritsuka observes a weakness in the female Zero Sacrifice that allows Soubi and Ritsuka to triumph in their next battle against the female Zeroes.
Ritsuka sleeps at Soubi's apartment to allow him to stay awake at night to meet with Septimal Moon. Soubi is present for Septimal Moon's interview of Ritsuka and Septimal Moon tells Ritsuka to ask Soubi about Seimei, telling Ritsuka that "Soubi knows everything". Soubi interrupts this session, pronouncing it cruel to tell Ritsuka that Seimei may still be alive and a different Seimei from the one Ritsuka knows. When asked, Soubi states emphatically that Seimei is dead.
Soubi walks Ritsuka back to his house after Ritsuka's mother discovers that Ritsuka is missing. He keeps Ritsuka from trying to console his mother and goes with Ritsuka to meet Fearless. Ritsuka and Soubi defeat Fearless who tell them that Fearless met with Seimei only last week. Soubi and Ritsuka return to Ritsuka's house, Ritsuka filled with doubt about Soubi, to find Ritsuka's mother unconscious.
Soubi meets Mikado Gomon of Moonless who has sent Tokino Fujiwara to meet with Ritsuka. The pairs meet up and return to Soubi's apartment where Moonless proposes that they form an alliance to take on Seimei. Soubi shows Ritsuka his parents' grave and tells Ritsuka how Ritsu raised him. Seimei appears then and tells Ritsuka that he is taking back Soubi and Soubi leaves with Seimei, putting Ritsuka in despair.
Later, Soubi accompanies Seimei and Nisei to meet Moonless where Seimei orders Nisei to engage with Moonless in a Spell battle by himself.
At first Soubi is mysterious, withholding information from Ritsuka and confusing him with his contradictory behaviour of stating that he'll do anything Ritsuka says but disobeying certain orders of his. Initially the relationship between Ritsuka and himself is full of conflict over misunderstandings and the refusal to communicate.
Soubi, although not a masochist, is willing to bear great amounts of pain if it is "punishment". He explains to Ritsuka that there is, for Soubi, a difference between violence and punishment. To Soubi, punishment is violence with an intention to discipline and is wholly acceptable and even desired from his master. This is something that Ritsu inculcated in him when he was young. Ritsu used to whip him and tell him that he must learn to bear pain for his Sacrifice without making a sound.
Soubi is abnormally good at hiding his emotions, but at times, his weaknesses and inner conflicts show through his mask. He's been groomed to become an object, and has lived his life as an object, only attaching importance to the person who is his "master" and insisting that he be given orders to follow. Soubi is very distant towards other individuals who are not his master, but has eventually formed relationships with other characters such as Yuiko, the male Zeroes, and Kio.
Kio often asked him to go out with him and other college students, and talked to him during class, but Soubi only took a genuine interest in Kio after seeing a butterfly tattoo on Kio's back and learning of the vow Kio made when he had it done. Despite finding Kio annoying, Soubi cares for him a lot, and when Kio was kidnapped, Ritsuka and Soubi rushed to his rescue.
After fighting the male Zero pair, Natsuo and Youji, he offered to let them live with him because they had nowhere to go, despite their cruelty in driving a nail through his hand during the battle. They soon grow close to each other, and the two help Soubi and Ritsuka on several occasions. Natsuo and Youji even choose to live with Soubi over Nagisa after their battle with Bloodless.
On several occasions Soubi teases and makes Ritsuka’s teacher Hitomi Shinonome cry. Though his dislike for her is genuine, it was for her safety that he fought the male Zero pair in the first place. Later, when Hitomi finally finds the courage to call him to ask about Ritsuka, who has been absent from school for several days, Soubi tells her about Misaki Aoyagi's abuse of Ritsuka, and tells Hitomi that there's nothing she can do about it anyway, expecting her to pull back after hearing this. Instead, Hitomi passionately states that although she is scared of Ritsuka's mother, she will confront Misaki for Ritsuka's sake. To this, Soubi replies that he may like teachers a little better because of her, indicating a grudge or mistrust towards teacher figures that most likely stems from Ritsu's treatment of him.
Soubi displays a sense of inferiority when compared to other Fighters with a name because he is a blank Fighter that has no destined name. Ritsu assures him that this is a source of versatility and thus strength, but Soubi seems to regard it only as evidence that he can be easily cast off.
Soubi has deep abandonment issues - first his parents died and left him to the abusive Ritsu, then Ritsu who started a sexual relationship with Soubi and left Soubi with the impression that he would become Ritsu's Fighter gave him to Seimei, and then Seimei died and left Soubi to Ritsuka only to reveal that he was alive and hadn't bothered to tell Soubi of his plans and had selected a new Fighter. When Nisei introduces himself and reveals that Seimei is still alive, and the male Zeroes return to Nagisa, Soubi reveals to Kio that Soubi believes he is a thing that is "easily thrown away".
Because of these attachment issues, Soubi is also entirely willing and capable of murdering Nisei for being Seimei's true Fighter.
Soubi is childish and cannot manage his emotions well. He gives in to anger by resorting to physical violence with Sleepless and Nisei Akame, and responds to upsetting events by withdrawing into sulks or a passive state. Upon finding out that Seimei has taken on Nisei as his Fighter to replace Soubi, Soubi goes to Ritsuka and begs him to run away together.
Soubi regards Seimei's word as "absolute", shown when Seimei tells him that Soubi will one day play Wisdom Resurrection and that he (Seimei) will one day be killed by Septimal Moon, both these things coming to pass. And even though Seimei fakes his death without telling Soubi and abandons him for another Fighter, Soubi still follows his command to break the window at Seven Voices Academy to allow Seimei to escape, even when doing so is against the wishes of Ritsuka and everyone at Seven Voices Academy.
However, Soubi also gives indications of believing this less now. He tells Ritsuka during the Seven Voices Academt incident that "not everything Seimei says is the truth" and when Seimei takes him back from Ritsuka, Soubi is clearly unhappy about it.
- Soubi is sometimes looked down upon by fans due to his poor social skills when interacting with other characters in the series.
- In the anime character commentary(also known as SD Theater) featured in the Loveless Complete Series DVD Collection, Soubi has a habit of being extremely hyper and outgoing. This often results in Ritsuka questioning if Soubi is actually the "real Soubi". Soubi then replies stating that he doesn't feel the need to be serious since he is not required to take on his role as a Fighter during the commentary.
- In the third episode of SD Theater, Soubi claims that he is practically blind without his glasses; he says that he can't "see a lion that is two kilometers away" without his glasses. Ritsuka then objects saying people with normal eyesight can not achieve that level of eyesight either (with or without glasses). In conclusion, it is still questionable whether Soubi's eyesight is glasses dependent or whether his glasses are simply an aesthetic choice.
- Soubi and Ritsu share some similarities: they both smoke, they wear glasses, Ritsu takes an interest in pinning butterflies and Soubi is associated with the motif of butterflies, and at one time Ritsu had long hair like Soubi. It also mentioned by Ritsuka that Ritsu and Soubi talk alike.
- In one of the more recent chapters, Soubi has been revealed to be able to interfere with memories, as he uses his powers to make Hitomi Shinonome believe that she is meeting Youji and Natsuo Sagan for the first time when they transfer into Ritsuka's class. This has caused fans to speculate that Soubi may have been the one to cause Ritsuka's amnesia on Seimei's orders.
- Soubi is used to reveal much of the mechanics of a Fighter-Sacrifice pair. For example, Soubi is considered despicable by other Units for fighting for another Sacrifice after his own Sacrifice has died, and for not killing himself after Seimei died. Any damage inflicted on Ritsuka will be four times the normal damage as Soubi and Ritsuka do not share a name. The carving "Beloved" on Soubi's neck also bleeds whenever he battles because he is not battling for Seimei.
According to databook(s):
- His favorite colors are red and gold.
- His hobbies are cooking, drawing/painting, international cultural heritage.
- He doesn't have any particular preferences in food.
- He has 3 piercings, two in his ears and another that is not specified.