The eldest Ayanokōji child, the genius Kazuya helped his grandfather with the experiments intended to create Absolute Angels. While Kyoshiro believed that Kazuya sacrificed himself to avoid the catastrophe ten years earlier, Sōjirō considered him a fool. Kazuya resembled the prince in Kū's dreams, because her dreams were, in truth, memories of her time in the lab being developed by him. He reappears after being missing for ten years and presumed dead.
The real Kazuya- not the one from his younger brother's childish fantasies -appears to be an evil, insane person with egomaniacal tendencies, who behaves and speaks in an overly theatrical manner. His saving of his younger brother Kyoshiro was completely unintentional; Kazuya having accidentally pushed him into an escape pod when he wanted him out of the way. He generally looks down upon his siblings, claiming that nothing they have done has held any meaning and frequently preaching they ought to be more like he, himself. In the past, he purposely tormented his younger sister Mika by stealing her first love away. In the present, he brutally abuses his brothers and murders his sister.
Upon his return, Kazuya explains that it was he who intentionally awoke the Absolute Angel Cielo against the counsel of his grandfather Reitarō, thus bringing about the 'Seven Days of Hot Snow'. His exposure to a deluge of Cielo's Mana caused him to be transformed into something other than human, capable of using Mana himself. He demonstrates inhuman powers, which usually manifest as thorny vines growing from his hands, with which he can cause vast destruction. He can apparently also teleport and is capable of draining Mana out of an Absolute Angel with a kiss, a clear indication of his inhumanity. His abduction of the Absolute Angels, including Kū, all serve his goal of reuniting them into the original Cielo. To objections voiced both by his grandfather and his youngest brother, that the Absolute Angels could destroy the world if unleashed, he has replied that the destruction of the world is insignificant in comparison with the Absolute Angels' beauty. Kazuya ends up in a wheelchair at the end of the series and appears to be suffering from some mental damage; he is being taken care of by Valteishia.
- He appears to bear some resemblance to Tsubasa from Kannazuki no Miko.