Sachio, a resourceful street child, scrapes by in the Restricted Area by pilfering mechanical parts with his gang, bartering them to a local mechanic for red candy or petty cash. Orphaned after his engineer father’s mysterious death under the Shirato Group’s employ, he harbors simmering hatred toward the corporation, fueling his early distrust of authority and defiant survivalist ethos.

Rescued by Joe during a clash with the mechanic, he attaches himself to Joe and coach Gansaku Nanbu, leveraging his mechanical expertise to maintain gear and devise fight strategies. His loyalty masks a craving for validation, framing Joe’s ascent in Megalonia as a beacon for the marginalized.

Post-season one, he settles at Gym Nowhere with Joe and Nanbu until a tsunami destroys their home, forcing him into Yukiko Shirato’s orphanage amid strained ties with Joe. Five years later, he fights as an underground Megaloboxer in Fujimaki’s ring, plagued by losses from his limited athletic talent. Desperation and bitterness toward Joe—whom he accuses of abandoning Nanbu during the coach’s fatal illness—drive his reckless bouts.

His arc spirals from fiery defiance to jaded complexity: childhood hero-worship curdles into resentment under betrayal and the Restricted Area’s brutality. Yet season two traces his fraught efforts to grapple with his past—his father’s shadow, his allegiance to Joe’s legacy, and the gnawing consequences of choices carved from survival.