The eldest son of the Ayanokōji family and a genius, who was involved in the experiments to create Absolute Angels alongside his grandfather. Kyōshirō believes that he sacrificed himself in an attempt to quell the great catastrophe of ten years prior to the main time line of the story. Sōjirō, however, has stated that for all his positive traits, Kazuya was a fool. Mika has added to this that Kazuya was as ruthless as a demon behind his eternal smile and tendency to always be victorious in any challenge. Kyōshirō looks up to him and makes every effort to emulate him, something his brother Sōjirō has advised him against and which arouses Mika's hatred for him.

Kazuya resembles the prince from Kū's dreams even more strongly than Kyōshirō does; her dreams of the prince are in fact memories of the time she spent in the lab, being developed by Kazuya and the other scientists.