She is the sacrifice of the female Zero pair. In contrast to Kouya's solemness, Yamato has an outgoing, vivacious personality. Their relationship is the focus of a mini-arc in volume 4.  The relationship Yamato has with Kouya is a passionate and dedicated one that becomes unexpectedly stressful for Yamato in volume 4. Her marking, which she interprets as physical proof of her relationship with Kouya, slowly fades without warning. In a blatant reference to the homosexual nature of their relationship, both girls' markings—a 0 with a line through the middle—are on the same place: their breast.   (Source: Wikipedia)

Twelve-year old Aoyagi Ritsuka is left with his insane mother as his only family when his brother, Seimei, is killed suddenly. After moving to a new school, he meets Agatsuma Soubi, who claims to have known his brother. Ritsuka eventually discovers that Soubi and Seimei used to be a fighting pair, whereby Soubi was the "Fighter" and Seimei was the "Sacrifice". Now that Seimei is gone, Ritsuka has inherited Soubi, who will become his "Fighter". After learning that Seimei was killed by an organisation known as the "Seven Moons", Ritsuka decides to investigate into his brother's death, with the sometimes useless help of Soubi, along the way.
 
(Source: Anime News Network)

__Non-Anime Roles:__
- Kula Diamond in _King of Fighters_ game series

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Aoyagi Ritsuka is one troubled 6th grader. Two years ago, he mysteriously lost his memory and developed an alternate personality. His mother constantly berates (and occasionally beats) him, asking what happened to the "real" Ritsuka, and his only defender and friend, his older brother Seimei, was recently killed. 
Suddenly, Seimei's apparent "friend," Soubi, appears. Soubi tells him that Ritsuka's true name is "Loveless," and that Soubi was assigned by Seimei to take care of Ritsuka and protect him. Ritsuka soon gets sucked into a bizzare underground society where teams of people, a "Fighter Unit," and a "Sacrifice," battle using elaborate and beautifully-phrased spells. 
He seeks both acceptance of himself, and the answer to the mystery of who killed his brother and why. 
(Source: Tokyopop)

Yamato Nakano