Japanese name:	坂上江夜
Nickname:	Zero

She is the fighter of the female Zero pair. Until they met, both pairs believed they were the only Zero pair to exist. Both teams are experiments created by Nagisa at different times, and the female pair is presumably the first generation. Despite their unusual background, both Koya and Yamato attend high school, although different ones.
In an interesting demonstration of the series' fictional world's rules and expectations of society, Koya wears fake cat ears despite not being a virgin. Yamato often teases her about it, particularly because Yamato doesn't try to hide her status. They lost their "ears" to each other. Shortly, after they first met, Kouya slit her wrists just to "know how it would feel", since zeroes can't feel pain.

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