Koichi works as a postman on Shio Island and has shared a childhood history with Ikuko, their bond stretching back to their earliest years. His quiet, unspoken affection for her lingers beneath their exchanges, yet he chooses to prioritize steadfast support over confessing his feelings. When Ikuko and Momo relocate to the island, he becomes a consistent ally, easing their adjustments through practical aid and emotional steadiness.

During a life-threatening asthma crisis involving Ikuko, Koichi braves stormy weather to cross the island’s newly built bridge, urgently securing medical help. This decisive act underscores his dependability in emergencies and his dedication to safeguarding the family’s well-being.

Beyond physical assistance, he gently mediates strained moments between Momo and Ikuko, offering clarity when grief or miscommunication clouds their relationship. His deep knowledge of the island’s terrain, residents, and networks allows him to seamlessly connect the family to essential resources, solidifying his role as both a pragmatic problem-solver and a subtle emotional anchor in their lives.