Arisu Terui 照井有栖

She is a second-year student in Class 4 at Fujinari Academy High School and serves as the treasurer for the student council. Her strongest subjects are biology and chemistry; she has even won awards in competitions. Although she also heads the Chemistry Club, she is effectively the only active member, as the only other person on the roster—Takaka—merely lends her name to the club.

Raised in a wealthy, refined family, she appears at first glance to be a gentle, traditional beauty—the sort raised in the lap of luxury—but she is also skilled in martial arts. She excels in both academics and athletics, making her arguably the most formidable character in the story.

She is feared throughout the school for her aggressive nature—she wouldn't hesitate to deliver a kick to the groin right off the bat—and her sharp, unsparing tongue; her presence on stage at a school assembly is enough to silence all chatter instantly.

She ruthlessly slashes club budgets for the coming term, yet because she always conducts thorough research beforehand, no one dares to challenge her decisions. Students fear her, calling her the "Psycho Flat-Chested Youkai." She treats even the faculty without mercy if issues arise, effectively making her the school's "final boss."

She maintains what she believes to be a good relationship with her cute junior, Ume Mizunoe; however, Ume’s first impression of her was that she was a "scary person." In fact, whenever Ume encounters a suspicious incident on campus,she instinctively lists his senior as a potential person of interest.