Akako Onigashima is a student at Itan Private High School. She has bright red hair cut in a chin-length bob, featuring a thin ahoge on top and a single visible fang on her left side. When angered, her appearance shifts dramatically, with the ends of her hair spiking upwards like horns. Typically cheerful, friendly, and easygoing, Onigashima interacts positively with classmates and shows an accepting nature, readily befriending Hitohito Tadano. However, minor daily frustrations—like tangled headphones, untied shoelaces, traffic delays, or spilled drinks—trigger intense anger. This volatile state is marked by a visible "Monster Meter" indicating escalating rage levels, making her unpredictable. Known methods to calm her include eating, sleeping, bathing, or venting frustration at batting cages. Komi Shouko later discovers a fifth method: directly assisting Onigashima with the source of her irritation. She shares a long-standing friendship with Yamai Ren since elementary school. Yamai understands her temper and often avoids direct intervention during outbursts, describing the anger as uncontrollable in school. Onigashima refers to Yamai by her given name without honorifics, reflecting their closeness. A significant moment occurs when Komi intervenes during one of Onigashima's anger episodes. After Onigashima nearly drops tangled headphones into a sink, Komi retrieves and meticulously untangles them. This act of kindness moves Onigashima to tears, instantly calming her. In gratitude, she invites Komi, Tadano, Najimi, and Yamai to a batting cage, where she demonstrates exceptional batting skills by hitting a home run, strengthening her bond with Komi. Baseball serves as her primary hobby and emotional outlet, with batting cages her preferred location for stress relief. She also enjoys music, though headphone tangles frequently cause irritation. Her name, Onigashima Akako (鬼ヶ島 朱子), translates to "Red Girl from Oni Island," referencing the Japanese folktale of Momotarou, aligning with her demonic visual motifs during anger.