Hirono Kusuhara attends school with Nao Takanashi as both her classmate and friend. She frequently appears alongside Haruka Katō within their shared social circle. Acting as Nao’s confidante, Hirono observes and questions Nao’s persistent romantic interest in her adoptive brother, Shūsuke Takanashi. She similarly voices skepticism regarding Iroha Tsuchiura’s aggressive pursuit of Shūsuke, cementing her role as an observer of these central romantic tensions.

Hirono participates in group activities, such as accompanying Nao and Haruka to discreetly follow Shūsuke and Iroha during a date outing. During this surveillance, she witnesses interactions and resulting tensions among the group. At school, she is present during casual moments like lunch breaks or gym-class preparations, where she occasionally observes or reacts to underlying rivalries, such as sensing impending confrontations between Nao and Iroha.