Shu exists across two timelines under distinct identities. In the modern era, he is Shuhei, a young neighbor child cared for by Suzuko, identifiable by a surgical scar from an appendectomy. A catastrophic gas explosion during their walk home displaces both through time.

Arriving years earlier than Suzuko in Japan's violent Sengoku period, he is discovered as an infant by villagers. Raised as Shukumaru, he matures into a protective yet rough-mannered community member, scavenging battlefields and defending against bandits. He forms a deep bond with a girl named Suzu, believing her to be his biological sister.

Upon rescuing Suzuko later, Shukumaru immediately declares romantic interest and his intent to marry her, persisting despite cultural friction and even after Suzuko deduces their potential sibling link through Suzu's identity. His survival demands constant adaptability and resilience amidst warfare and scarcity.

A critical revelation confirms Shukumaru and Shuhei are the same person, separated during the initial displacement, with the appendix scar providing definitive proof. Briefly returned to the modern era with Suzuko, he expresses a preference for his integrated life in the past. The narrative concludes with his voluntary return alongside Suzuko to the Sengoku period, where they plan to marry and remain permanently.

His existence anchors the story's time-loop: as an infant, he witnesses Suzu (Suzuko's younger self) vanish during a village fire, foreshadowing his own displacement. This establishes fire as the catalyst for temporal movement affecting both characters.