Kyōsuke Kazama, a Tokyo high school student, befriends Haruto Kirishima after Haruto moves to the city. Outgoing and cheerful, he frequently engages in lighthearted teasing, especially targeting Asuka Mishima's regional dialect. His driving passion is motorcycle racing, fueled by an ambition to turn professional. This goal is complicated by an unspecified terminal illness confining his lifespan to roughly one year. Despite this prognosis, Kyōsuke clings to a positive outlook, prioritizing a life without regrets.

He shares a history with Yuzuki Eba dating back to middle school, marked by his repeated confessions of feelings and her consistent rejections. Following Yuzuki's return to Tokyo from Hiroshima, Kyōsuke confesses once more, this time revealing his terminal condition and asking her to be his girlfriend for his remaining time. Yuzuki agrees out of compassion, creating a complex romantic triangle involving Haruto, who still loves her.

Upon discovering Haruto and Yuzuki's past relationship, Kyōsuke encourages openness between them, initiating a respectful rivalry for Yuzuki's affection. Seeking to prove his worth beyond his illness and potentially extend his time with Yuzuki, he chooses a high-risk surgical procedure despite its fatal potential. The surgery initially seems successful, allowing him time to inscribe a message reserving sponsorship space on his motorcycle helmet for Haruto's future restaurant. However, Kyōsuke succumbs to postoperative stress and dies shortly after.

His death deeply impacts his friends. Yuzuki, burdened by guilt over influencing his decision for surgery, initially severs contact with Haruto. Kyōsuke's helmet, bearing the dedication to Haruto's culinary dream, becomes a symbolic keepsake displayed in Haruto's eventual restaurant. Kyōsuke's funeral acts as a catalyst, significantly widening the emotional distance between Haruto and Yuzuki for years before their eventual reconciliation.