Kaoru Matsunuma, the older sister of Sumio Matsunuma and a childhood friend of the protagonist, evolves into his romantic partner after he relocates to Karuizawa and moves in with her brother. Her narrative intertwines with Noriko Kuonji, a former bōsōzoku leader, as both women become pivotal influences guiding the protagonist’s journey toward self-discovery.

Her life in Karuizawa anchors the story’s relational conflicts, driven by family ties and layered connections. The obstacles she faces demand collaborative resolution, while her interactions with others position her as a mediator of tensions, underscoring themes of accountability and maturation through pivotal narrative dynamics.