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)