Kōsuke Kumagaya, an 11-year-old elementary student, shares a bond with Natsume Tonai reminiscent of siblings, forged within the walls of their soon-to-be-demolished apartment complex. Their once-close ties fray after the loss of Yasuji, Kōsuke’s grandfather and a pivotal figure in both their lives, leaving unresolved grief and simmering resentment. A critical argument—sparked by Kōsuke’s missed chance to bid Yasuji farewell—deepens their rift, compounded by the looming destruction of their childhood home.

Emotionally guarded, Kōsuke masks vulnerability with distance or irritation, his grief tangled in misplaced blame toward Natsume, whom he accused of overshadowing his bond with Yasuji. Yet beneath his stoicism lies a responsible, brave spirit, evident in his role as a forward on the school soccer team and his logical approach to challenges. Though initially dismissive of Natsume’s familial connection, a surreal supernatural event—trapping them in their drifting apartment—forces collaboration, nudging him toward introspection.

Clad in a striped white shirt, navy shorts, and sport shoes, his dark blue hair and perpetual frown mirror his inner turbulence. The ordeal unravels Natsume’s painful history of familial strife, compelling Kōsuke to confront his own guarded heart. Through shared struggle, he sheds resentment, embracing empathy and renewed loyalty, bridging their divide with tentative vulnerability and a resolve to mend what was broken.