Kosuke Kumagaya, a 12-year-old elementary student with dark blue hair and eyes, a slim frame, and a persistent frown, shares a sibling-like bond with Natsume Tonai, forged during their childhood in a condemned apartment complex. This bond shatters after his grandfather Yasuji—who embraced Natsume as family—passes away, and their clashing views on the building’s demolition deepen the rift.

Inherently logical and reserved, Kosuke masks his emotions, fueling misunderstandings. He resents Natsume for overshadowing his connection to Yasuji, especially after a heated argument robbed him of a final goodbye. Amid a surreal oceanic ordeal with classmates, he evolves into a courageous leader, prioritizing collective survival through pragmatic decisions while tentatively embracing vulnerability.

His journey hinges on reconciling with Natsume, whose guilt over past wounds strains their dynamic. Through shared trials in the drifting realm—confronting memories of the apartment and Yasuji’s influence—he begins grasping her struggles with familial instability and stagnation. A mysterious spirit tied to the building’s history prods his acceptance of loss, reshaping his perspective.

Clad in a black-striped white shirt, navy shorts, pale cyan socks, and soccer shoes reflecting his athleticism, Kosuke blends methodical problem-solving with emerging emotional insight. His arc traces a path from simmering resentment to mutual healing, anchoring themes of release and repaired bonds amid adversity.