Another one of Ayumi's friends, Koyoi has an open brother complex. Once Misao starts hanging around, she, along with Chikura, stop walking home with Ayumi. She is also in the same grade as Ayumi.

Since her brother likes Misaki, a popular and attractive girl, she regards her as a "Final Boss" that she must defeat. She recruits Ayumi along the way once she learns that her love, Mamoru also has a crush on Misaki. She also quotes that Misaki's chest is a "truly evil power."