Koichiro Mutoh is a 44-year-old Japanese man, married to Mari Mutoh. They have two children: Ayumu, aged 14, and Go, aged 7. He works as a lights engineer for a stage production company.

Physically tall and well-built with short black hair, Koichiro typically wears construction work attire or casual clothing like a plain green shirt and jeans. His profession becomes critical immediately after a catastrophic earthquake when he illuminates trees near his home using colored stage lights. This guides his family to safety at a hilltop shrine, facilitating their initial reunion.

Upon reuniting with his son Go, whose eyes are bandaged after being struck by earthquake debris, Koichiro assesses the wounds. He determines the injuries are limited to the eyelids, noting Go's gaming reflexes may have helped him avoid worse harm. Using a stapler from his toolkit, Koichiro patches the cuts.

As flooding forces survivors to evacuate the shrine, Koichiro advocates traveling westward based on information from Go's online gaming contacts, countering suggestions to head east. During the journey, he maintains optimism, reassuring Ayumu after she donates their remaining water to an elderly couple. His survival skills surface when he kills a wild boar for food. Later, aware of Ayumu's refusal to eat meat, he risks digging for yams in a restricted area to provide her with sustenance.

While excavating yams, Koichiro's shovel strikes an unexploded wartime bomb buried in the soil. It detonates instantly, killing him. His limbs and flesh scatter onto his family and fellow survivors, leaving them in profound shock and grief.

His background as a resilient provider and protector defines his character arc. His actions consistently prioritize family safety, from guiding them with professional expertise to sourcing food under dire circumstances. His abrupt demise highlights sudden loss and the precariousness of survival during large-scale disasters.