Masa leads a group of orphans struggling to survive in Hiroshima three years after the atomic bomb took their parents. They scavenge for food and evade authorities. While originally depicted as a yakuza opposing Ryuta in the manga, the anime reimagines him as an orphaned teenager leading other parentless children. Masa and his gang clash with Gen and Ryuta, disrupting their shoe-shining business and drawing police attention, leading to an attempt to institutionalize the orphans.

Masa fiercely desires independence, explaining they fled the orphanage due to its strict rules. He prioritizes living freely without external control. To secure necessities, he organizes the group to pickpocket in the black market. Despite initial hostility, Masa develops a bond with Gen and Ryuta after fighting off the police together. He allows them to join his group and take shelter. Masa crucially rallies the orphans when Gen proposes building a new home from salvaged materials, an effort providing stability and a sense of family.

Demonstrating loyalty, Masa participates in risky efforts to help Gen's ailing mother. He joins Gen and Ryuta in an unsuccessful attempt to steal copper from a shipyard to fund medicine. Later, he assists in collecting copper bullets from an abandoned firing range to sell for scrap. After Gen's mother's death, Masa remains a supportive figure, seen playing with Gen, Ryuta, and the orphans, integrated into their makeshift family. Scholarly analysis suggests Masa embodies resistance against authoritarian structures, like the Imperial system, through his rejection of the orphanage and leadership in forming an independent community, aligning with themes of self-determination and societal rebuilding.