Yumiri Hoshino, a student sharing Kouichi and Kazuki's class, possesses shoulder-length brown hair and brown eyes. She begins as an intensely shy individual with few friends, frequently found absorbed in books and hesitant to engage in social activities like singing publicly. Meeting Kouichi sparks a transformation; she becomes more sociable and brave, crediting their relationship for this growth. Her observant nature allows her to detect others' concealed feelings, and she actively strives to build connections.

Before transferring schools, Yumiri serves on the library committee. Though not an official member, she assists the movie club with their projects. This involvement includes attending Mao's welcome-back party from Paris. Overwhelmed by karaoke, Yumiri retreats to the bathroom, where Mao joins her for a private conversation. Mao encourages Yumiri to pursue her relationship with Kouichi, positioning herself as a confidante.

Within her social circle, Yumiri's role centers on connecting characters and supporting their emotional developments.