Koyoi Bessho is a middle school student and a close friend of Ayumi Arihara, with whom she shares the same grade. She is recognizable by her long brown hair and pink eyes. The most conspicuous aspect of her character is an open and intense brother complex directed at her older brother, Yoshihiko. She displays excessive affection toward him, much to his annoyance, and has no romantic interest in boys her own age. Her single-minded devotion peaks when her father is transferred for work, as she is overjoyed to be living alone with her brother.

Koyoi’s fixation on Yoshihiko leads her to view Misaki Yamamoto, a popular and attractive high school girl whom her brother has a crush on, as a rival. She dramatically labels Misaki a “Final Boss” that must be defeated and even calls Misaki’s chest a “truly evil power,” revealing a mix of jealousy and comedic exaggeration. When she learns that Ayumi’s love interest, Mamoru Zaitsu, also harbors feelings for Misaki, Koyoi recruits Ayumi into her personal vendetta against the older girl.

In the everyday social dynamic, Koyoi initially walks home with Ayumi and their friend Chikura, but after the intimidating high schooler Misao Zaitsu begins spending time with Ayumi, Koyoi and Chikura distance themselves and stop walking home with her. Her role in the story is primarily that of a comedic character whose exaggerated sibling obsession and anti-Misaki crusade add to the interlocking romantic entanglements. She does not undergo marked personal development, but her unwavering, if misguided, loyalty and her energetic scheming serve to illustrate the chaotic, overlapping web of first loves among the cast.