Koichi is the father of five-year-old Sōsuke and husband to Lisa. His seaman occupation demands long absences aboard the cargo ship "Kōshin Maru," shaping family dynamics as he communicates via Morse code signals from the vessel. During one voyage, Koichi observes the goddess Granmamare crossing the ocean, identifying her as the Goddess of Mercy. This encounter coincides with his ship's engine failure amid a supernatural storm triggered by Ponyo's magic, but Granmamare's presence restores the ship's function and secures its safety.

Koichi shares resilient bonds of affection with his family. Sōsuke voices concern for his father's safety during the storm, while Lisa shows frustration over delayed returns alongside steadfast loyalty. Their light-signal exchanges include Lisa playfully teasing Koichi. Sōsuke addresses him informally as "Koichi," reflecting their familial closeness.

In the story's climax, Koichi's ship faces peril from a magical tsunami and the descending moon. After Ponyo becomes human and natural balance returns, his vessel reaches port safely. He reunites with Lisa and Sōsuke during the community-led resolution of the flooding crisis.