Tsubame Koyasu, a widely popular third-year at Shuchiin Academy, navigates social circles with fairness and calm observation. Her role as Cheerleading Squad vice-leader showcases leadership and tenacity. Approachable and skilled at caring for others, she actively supports students facing social difficulties, like Yu Ishigami. Observing his struggles during a sports festival integration, she initiates contact to foster his inclusion, sparking a connection where Ishigami develops feelings for her.

Tsubame encourages personal growth; upon learning of Ishigami's interest, she indirectly motivates him to aim for a top 50 academic ranking. Though he initially falls short, this fuels his determination. Navigating romance, she sets a three-month deadline to answer Ishigami's confession, tying it to the school's lost-and-found holding period for a symbolic heart pendant. Hosting a Christmas party at her home, she invites both Ishigami and Miko Iino, ensuring inclusivity despite their tensions.

Their relationship progresses to significant physical intimacy, including sharing a bed. However, complications prevent it from becoming a lasting partnership. Her story explores teenage romance, social responsibility, and the impact of personal choices.