Until they met, both pairs believed they were the only Zero pair to exist. Both teams are experiments created by Nagisa at different times, with the female pair presumably being the first generation. Despite their unusual background, both Kouya and Yamato attend high school, though at different institutions.
Kouya, unlike Yamato, is very serious and does not take things lightly. She is reserved, though Yamato is the only one who can get her to open up.
Kouya meets Yamato when they are both young and are trained together to work as the Zero pair. As they grow up, her feelings for Yamato deepen until both girls fall in love with each other.
She is deeply in love with Yamato, despite trying to hide it from others. Her cold, aloof exterior serves as a shield for her true feelings; she is genuinely afraid of losing Yamato. This fear becomes so intense that she even throws up at the thought of being separated from her lover and partner.
It is evident that Kouya and Yamato have a physical and romantic relationship alongside their professional partnership as Zero. In an interesting reflection of the series' fictional world's rules and societal expectations, Kouya wears fake cat ears despite not being a virgin. Yamato often teases her about this, especially since Yamato herself does not try to hide her status.
