Fumi Ninno possesses green eyes and waist-length brown hair, appearing as a teenager. She is a member of the Ninno family, specifically one of Takeshi Ninno's stepsisters. Her unexpected return to the household adds further complications to the family's already strained dynamics. This return happens amidst existing tensions fueled by Takeshi's coercive relationship with his stepmother, Shino, and difficult interactions with his other stepsister, Shinobu.

Fumi's presence amplifies these familial conflicts. Takeshi actively attempts to hide his illicit involvement with Shino from both Fumi and Shinobu. Her role contributes to the narrative's focus on secrecy and destabilized relationships within the home. Character databases list her as "Fumi Ninno," though one synopsis source refers to her as "Fumino Oshino," indicating possible variations in spelling or surname attribution.