Kaya is a young woman whose sister tragically died at the age of 18, nine years ago, during the first illegal fighting tournament organized by Kiyoko Sunomata. Following this devastating loss, Kaya was taken under the wing of Ryuuji's father, who provided her with one of his houses to live in. However, she became emotionally withdrawn until Ryuuji asked her to take in Tsukasa and her brothers. It was then that Kaya started to smile again.
Her affection for Tsukasa runs so deep that she gifts her late sister's quipao to Tsukasa, who later wears it while infiltrating the second illegal tournament. Despite being aware that Tsukasa serves as Ryuuji's bodyguard, Kaya can't help but worry when Tsukasa gets involved in fights, risking her life. After the Kozuki siblings resolve the conflict between Iruma and Tenkou, Kaya moves into Iruma's apartment complex with them. She also requests Ryuuji not to spoil her any longer.
