Akako Onigashima is a first-year student at Itan Private High School, sharing Class 1-1 with Shouko Komi and Hitohito Tadano. She has short, wavy red hair cut in a chin-length bob with a thin ahoge on top, brown eyes, and a single visible fang on her left side. In winter, she adds a headband to her standard uniform. When angered, the ends of her hair spike upward like horns, mirroring her volatile emotional state measured by an internal Monster Meter. Typically, Onigashima is bubbly, cheerful, and easygoing, greeting everyone warmly and readily befriending even unpopular students like Tadano. However, minor frustrations such as tangled headphones, untied shoelaces, traffic delays, or spilled drinks trigger intense, intimidating anger. Known methods to calm herself include eating, sleeping, bathing, or venting at a batting cage. Shouko Komi later discovers a fifth method: directly helping Onigashima resolve the provoking issue. An avid baseball enthusiast, Onigashima frequently visits batting centers and demonstrates significant skill, easily hitting home runs. She enjoys music, though tangled headphones are a known irritant. A pivotal moment occurs when Komi retrieves and meticulously untangles Onigashima’s headphones, moving her to tears of gratitude and instantly calming her. In thanks, Onigashima invites Komi, Tadano, Ren Yamai, and Najimi to a batting cage. Her long-standing friendship with Yamai dates back to elementary school, and she addresses Yamai by her given name without honorifics. Onigashima consistently appears within Yamai’s social circle alongside Himeko Kishi. Her name translates to Red Girl from Oni Island, alluding to the Momotarō folktale and her monstrous anger transformation. She was born on February 3rd, making her zodiac sign Aquarius, and ranked 22nd in a series popularity poll. While she shares Class 1-1 with Komi and Tadano only in the first year, she moves to a different classroom in their second year.