Kosuke Tsuda attends Kuranose High School as part of a tight friend group with Makoto Konno and Chiaki Mamiya. His father runs a clinic, reflecting his stable upbringing. Academically focused and organized, he favors library time or studying over his friends' spontaneity, yet actively joins their baseball games and catch sessions.

With black hair and glasses enhancing his studious look, Kosuke displays a responsible, level-headed nature, frequently noting Makoto's tardiness. Initially among Makoto's closest friends, their dynamic shifts as unspoken, unreciprocated feelings from Kosuke fade. He instead pursues romance with classmate Kaho Fujitani after she injures her ankle. Unaware of faulty brakes, he borrows Makoto's bicycle to rush her to his father's clinic.

This act sparks a pivotal incident: cycling toward a railroad crossing reminiscent of Makoto's past near-fatal accident, they nearly collide with an oncoming train despite Makoto's failed attempt to intervene. Chiaki Mamiya prevents disaster by using his final time-leap ability, saving them while sparing Makoto further anguish. Kosuke sustains his relationship with Kaho afterward. When Chiaki vanishes from their timeline, Kosuke accepts Makoto's explanation of his departure for overseas study.

Kosuke's character evolves subtly from a diligent trio member to Kaho's supportive boyfriend. His choice to borrow the bicycle inadvertently catalyzes critical narrative moments.