Ayumi Matsuoka, a second-year student at Houshou Private High School, leads the softball team as captain. Her athletic frame, fair skin, short brown hair, and piercing blue eyes complement her practical attire: a blue tracksuit with white stripes, white-and-blue sneakers, and a crisp white headband. Driven by an unwavering moral code, she fiercely protects others—particularly younger girls—and challenges group dynamics jeopardizing collective survival. A shrewd tactician, she maneuvers others’ obsessions to her advantage, such as exploiting Atsushi Kamijo’s hunger for authority to organize rescue efforts.

Though her social circle outshines peers like the introverted Mutsumi Oribe, Ayumi partners with Mutsumi during crises, respecting her entomological knowledge and survival acumen. Privately, Ayumi navigates discreet romantic and sexual attraction to women, a quiet facet shaping her interactions. This undercurrent surfaces when Mami Miura, a peer, initiates an intimate moment amid shared vulnerability.

Haunted by perceived leadership failures, Ayumi grapples with guilt over casualties and withering self-doubt after Mami condemns her crisis responses. Her bond with Mami evolves from mutual suspicion to fierce protectiveness, climaxing in Ayumi’s fatal sacrifice to shield Mami from giant firefly larvae—a martyrdom that fractures group morale in the manga.

Anime adaptations diverge, prolonging her survival as she strategizes clinic defenses against threats like parasitic outbreaks and colossal crabs. Her resourcefulness emerges in tactics such as deploying floodlights to incapacitate a rampaging dobsonfly. Whether facing mortal sacrifice or prolonged trials, Ayumi’s arc consistently weaves loyalty, resilience, and the psychological weight of commanding others in peril.