Koichi, a lifelong postman on Shio Island, shares a deep-rooted bond with Ikuko, tracing back to their childhood. His quiet, unspoken affection for her lingers, though unreturned, as he steadfastly supports her and her daughter Momo after they settle on the island following Kazuo’s tragic death. When a violent storm triggers Ikuko’s life-threatening asthma attack, Koichi urgently guides Momo across a perilous, newly built bridge—a passage mysteriously aided by supernatural forces—to secure medical aid, cementing his role as a dependable ally in crises.

His gentle empathy surfaces in moments with Momo, particularly as she confides in him about her unresolved guilt over her final confrontation with Kazuo. Koichi offers silent solidarity, anchoring her amid emotional turmoil. Community ties further define him, evidenced by his participation in the island’s tōrō nagashi ritual, where he joins neighbors in commemorating the departed.

Rooted entirely in Shio’s close-knit fabric, Koichi’s history remains anchored to the present, his actions consistently focused on providing stability for Ikuko and Momo as they navigate grief and transition. No elaborate past beyond his film portrayal dilutes this focus, reinforcing his quiet, unwavering presence in their lives.