Chiaki Mamiya, a high school student displaced from an environmentally ravaged future stripped of rivers, blue skies, and baseball, travels to the past with a singular goal: to witness the restoration of a painting by Makoto Konno’s aunt—a work erased from existence in his time. Adopting the guise of a cheerful, athletic teenager, he bonds with Makoto and Kōsuke Tsuda, joining their baseball games and carefree antics while concealing his profound grasp of time-leaping mechanics. This shared ability, triggered accidentally by Makoto through a walnut-shaped device, intertwines their fates.

Behind his easygoing facade lies strategic foresight. Chiaki covertly employs his final time leap to avert Kōsuke and Kaho Fujitani’s deaths in a train collision, knowingly stranding himself in the past. His unspoken affection for Makoto complicates their dynamic, as her repeated resets via time-leaping stifle his attempts to confess. Their relationship frays under unresolved tension, culminating in his sacrifice to rewrite the timeline and save her friends. In a poignant farewell, he discloses his origins and irreversible fate—vanishing permanently in the anime’s timeline, while the manga hints at survival.

Sporting orange hair, brown eyes, and an athletic frame, Chiaki balances humor with unwavering loyalty. He playfully teases Makoto yet fiercely shields her from criticism, subverting his "brawn over brains" image through calculated decisions and emotional depth. His influence spurs Makoto’s maturation, challenging her to accept responsibility for her choices. A final vow to await her in the future drives her commitment to preserving the painting, anchoring their bond across eras. Their story lingers on sacrifice, legacy, and the delicate balance between individual yearning and collective consequence.