Natsumi Kuzuryu

Fuyuhiko's sister was the late younger sister of Fuyuhiko Kuzuryuu. She was killed by a fellow student named Satou, who covered it up along with Mahiru Koizumi, blaming it on a serial pervert. Her death was avenged by Fuyuhiko, who in return killed Satou. In one of Fuyuhiko's free time events, it's revealed that his sister's title was the "Ultimate Little Sister."