Akako Onigashima is a first-year student at Itan Private High School, sharing Class 1-1 with Shouko Komi and Hitohito Tadano. She possesses short, wavy red hair styled in a chin-length bob featuring a thin ahoge on top; her brown eyes and a single visible left fang complete her look. When angered, her hair ends spike upward like horns, mirroring her volatile emotional state measured by an internal "Monster Meter." She wears the standard Itan uniform, adding a headband in winter. Typically, Onigashima radiates bubbly cheerfulness, greeting everyone warmly—including unpopular students like Tadano—earning her a reputation as the highly desirable "number one friend everyone wishes they had." However, minor frustrations trigger extreme, intimidating anger, causing even close friends like Ren Yamai to avoid her. She manages these outbursts through four methods: 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, demonstrating significant skill by hitting effortless home runs. She enjoys music, though tangled headphones are a known irritant. Her long-standing friendship with Ren Yamai dates back to elementary school, marked by casual address without honorifics. She consistently appears within Yamai's social circle alongside Himeko Kishi, participating in group activities supporting Komi's quest for friends. A pivotal chapter highlights her generally easy-going nature contrasted by extreme reactions to minor annoyances. Komi untangling her headphones moved Onigashima to tears of gratitude, prompting her to thank Komi, Tadano, Yamai, and Najimi with a trip to the batting center. Her name, Onigashima Akako (鬼ヶ島朱子), translates to "Red Girl from Oni Island," alluding to the Momotarō folktale and her monstrous anger transformation, feared even by Yamai. She ranked 22nd in series popularity polls. While interacting with broader groups like Omoharu Nakanaka's, her core relationships remain Yamai and Kishi. Academically, she shares Class 1-1 with Komi and Tadano only in the first year; she moves to a different classroom in their second year. Born February 3rd, her zodiac sign is Aquarius.