Ayumu Mutoh, a 14-year-old Filipino-Japanese teen, resides in Japan. Her mother Mari hails from Cebu, Philippines; her father Koichiro is Japanese, making Ayumu and her younger brother Go biracial. Pre-disaster, Ayumu trains rigorously as a track and field athlete aspiring for the Olympics, wearing Onitsuka Tiger running shoes. The family primarily speaks Japanese at home, though Go occasionally uses English phrases from online gaming.

Initially naive and routine-bound, Ayumu experiences profound trauma when disaster strikes. She openly expresses grief, fear, and rage amid mounting casualties, questioning her mother’s calmness. Her emotional rawness positions her as an audience surrogate for processing loss. Early in the crisis, she sustains a deep leg cut escaping a collapsing locker room. This untreated injury progressively worsens, becoming a physical burden.

During survival, Ayumu develops resilience and fierce protectiveness toward Go. Separated from their mother and adrift at sea, she maintains hope, signaling SOS via flashlight in Morse code (three short, three long, three short) and blowing a whistle for rescue. A bioluminescent plankton display reminds her of her father’s comforting lights, reinforcing family bonds. In Shan City, she thwarts an attempted rape with a decisive headbutt.

She aids Toshio Onodera, a near-paralyzed scientist, by deciphering his Morse code communicated through eye movements and thumb signals, facilitating critical data transfer about Japan’s geological fate. Her leg infection deteriorates severely, culminating in amputation of her left leg. In the epilogue, Ayumu adapts to a prosthetic limb. Her journey deepens her appreciation for familial bonds and endurance through loss, framing her maturation amid adversity.