Katsuma Shimano, a young resident of Nabu Island, grows up under the care of neighbors following his mother’s early passing and his father’s work-related absences. He maintains a tight-knit relationship with his older sister Mahoro, joining her in mischievous plots to test heroes’ response times, though his demeanor remains more remorseful and contemplative compared to her sharp skepticism. Though Mahoro discourages his idolization of heroes, Katsuma clings to fervent admiration, notably for Edgeshot, symbolized by an enamel pin on his messenger bag—a prize from a magazine raffle. His Quirk, Cell Activation, accelerates cellular regeneration in others, operating across all blood types—an advancement over his father’s limited version. During Nine’s assault on Nabu Island, this power proves critical as Katsuma pushes himself to heal gravely wounded heroes Izuku Midoriya and Katsuki Bakugo, teetering on collapse from overexertion. His selflessness intensifies when he volunteers to relinquish his Quirk to safeguard the island, halted only by Izuku’s counsel to cherish his own worth. Katsuma’s arc pivots on transcending shyness to act decisively in crisis. When Nine targets Mahoro, he charges forward despite mortal danger, echoing Izuku’s own history of defiant bravery. This moment, paired with Izuku’s guidance, cements his ambition to embrace heroism despite his Quirk’s lack of offensive utility. Subsequent encounters with Edgeshot and observations of urban hero work signal his sustained dedication to growth. Visually, Katsuma’s curly light-brown hair, freckles, and simple attire—blue overalls and a sun hat—mirror Izuku Midoriya’s design, thematically underscoring shared resilience. His story parallels Tenko Shimura’s childhood tribulations, diverging through supportive influences that steer him toward hope rather than despair. Personal artifacts like handmade floral bracelets from his late mother anchor his familial bonds. His dynamic with Mahoro, initially defined by her protective wariness of heroism’s risks, deepens through their ordeal during Nine’s siege. Post-attack, both siblings unite in gratitude toward the heroes, solidifying Katsuma’s resolve to forge his own heroic path despite inherent dangers.