Kousuke Tsuda, a 17-year-old high school student sharing a classroom with Makoto Konno and Chiaki Mamiya, balances diligent academic ambition with quiet compassion. His methodical approach to life—fueled by aspirations to become a doctor—clashes with Makoto’s impulsivity, as he meticulously weighs decisions against their consequences. Beyond textbooks, he volunteers with a group aiding the elderly and ill, a commitment that intertwines his path with Kaho Fujitani, a shy classmate whose grandmother he assisted. Their relationship blossoms after a clumsily orchestrated "crash into hello" engineered by Makoto, blending admiration and happenstance.

As the trio’s steadying force, Kousuke mediates tensions between Makoto’s spontaneity and Chiaki’s intensity, embodying the strategic "brains" to counterbalance their emotional extremes. Though reserved, fleeting glimpses of unspoken affection for Makoto surface, lingering beneath his pragmatic exterior. A defining moment arises when he unwittingly borrows Makoto’s defective bicycle, nearly costing him and Kaho their lives at a train crossing. This near-tragedy not only exposes his unwavering trust in friends but also triggers Makoto’s reckoning with the ripple effects of her time-leaping choices. His survival, secured by Chiaki’s sacrifice, cements the trio’s intertwined destinies.

Kousuke’s narrative arc remains anchored in steadfast dedication to medicine and service, with his gentle romance with Kaho underscoring his quiet reliability. His journey mirrors themes of accountability and the quiet yet profound impact of everyday choices, reflecting resilience over reinvention.