Mari Muto, the Filipino-Japanese mother of the Mutoh family, is married to Koichiro Mutoh. They have two children: Ayumu, a 14-year-old track athlete, and Go, a younger son fascinated by video games. Her mixed heritage forms part of her background, and she works to support her family before the disaster. Mari relies on a surgical support device for a heart condition, a consistent medical detail that significantly limits her physically throughout the survival journey.

During the initial earthquakes, she reunites with her family at a hillside shrine, comforting Ayumu after the teenager expresses guilt over abandoning teammates. A recurring trait is Mari's insistence on taking family photographs during moments of respite amid the chaos. This habit serves to preserve memories and maintain hope, initially seeming incongruous but later reflecting her role as the family's emotional anchor. She faces discrimination when Japanese nationalists deny the family passage on evacuation boats, prioritizing "pure" Japanese citizens and highlighting racism tied to her identity.

Mari's character development involves managing grief after Koichiro's death, protecting her children from external threats like cults and violence, and reconciling with Ayumu after conflicts typical of their strained mother-teenager relationship under extreme circumstances. She demonstrates resilience in leading the family forward despite her deteriorating physical health.

Her death occurs during the escape from Japan. Following the failure of her heart device, Mari sacrifices herself to save Ayumu from drowning in the ocean. Unable to swim effectively due to her medical condition, she drowns shortly after ensuring Ayumu's safety.