Kōichi Ōno is one of two male students in Mayu Morita's class who, alongside Ryūta Kokubo, habitually observes interactions between Mayu and her female friends. He consistently interprets these ordinary exchanges through a yuri (girls' love) lens, misconstruing them as romantic or suggestive scenarios. This misinterpretation serves as a recurring comedic element.

Kōichi functions exclusively as part of this observer duo. The narrative, across the manga, original video animations (OVAs), and anime television series, provides no individual storylines, personal background, character development, exploration of his family life, or personal motivations. His established role remains solely that of projecting fictionalized romantic narratives onto platonic interactions among female classmates.