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 Kuzuryuu's sister title are Ultimate Little Sister.