Fumika Mitarai holds the position of class president at St. Mihailov Academy. She possesses a petite frame with chin-length black hair obscuring the left side of her face and brown eyes. Her measurements are 63 cm bust, 51 cm waist, and 67 cm hips, with her notably flat chest causing significant personal insecurity and distress compared to more developed peers. She wears a modified high school uniform tailored to her stature and frequently adorns a cross-shaped brooch on her head's right side.

Despite her leadership role, Fumika exhibits a shy, timid, and reserved personality marked by low self-confidence. This meekness leaves her vulnerable to bullying from classmates like Miyuri and Hana, who mock her with derogatory nicknames such as "Toiletten-Fuu-chan" based on her surname's characters. She maintains a polite, quiet demeanor and shows kindness to individuals like Mafuyu Oribe and Tomo Yamanobe when others do not.

Her character development centers on unspoken romantic feelings for Alexander Nikolaevich Hell, whom she affectionately calls "Sasha." This infatuation motivates her gradual confidence growth, compelling her to confront threats like Eva Silver during a school invasion. Her desire to prove herself worthy of using Sasha's nickname fuels her efforts to overcome timidity.

Fumika harbors a hidden perverse streak, noted as the "most perverted of all the girls" in the series without explicit examples. She demonstrates exceptional skill at bowling, highlighted as an unusual hobby. Her surname, Mitarai, combines the characters for "your hand" and "washing," while her given name, Fumika, incorporates the character for "history."