Murakami Mamoru is the son of newspaper reporter Atsushi Murakami and Tomoko Murakami. The entire family becomes targeted by the alien invader Goa, who strands them in a prehistoric jungle. There, Goa annihilates a dinosaur before their eyes as a warning prior to his planned Earth invasion, establishing Mamoru's direct involvement in the conflict.

Following this experience, Mamoru encounters the golden giant Magma. He is taken deep beneath a volcanic island to meet Earth, the planet's creator. Intending to counter Goa, Earth reveals the creation of three rocket-people: Magma, his wife Mol, and their son Gum. Gum is explicitly modeled after Mamoru's appearance at Magma's request, reflecting their connection. Earth entrusts Mamoru with a special whistle: one blow summons Gum, two blows summon Mol, and three blows summon Magma for assistance during crises.

Mamoru subsequently works alongside the rocket-people to thwart Goa's repeated invasion attempts. Goa employs various tactics, including dispatching dinosaur-like space monsters and human-mimicking infiltrators called "Hitomodoki" or "Modoki Men." Mamoru utilizes the whistle to summon Magma, Mol, or Gum when these threats emerge, positioning him as an active participant in Earth's defense. His actions frequently place him and his family in danger, especially as Goa seeks media attention, leading to scenarios where they are caught in crossfires or directly targeted.

This role remains consistent across the original manga and the 1993 OVA adaptation. Mamoru maintains his relationship with the rocket-people and reliance on the whistle during confrontations. His background, abilities, and narrative function show no significant alterations in other official media. He makes a brief, non-speaking appearance in the *Astro Boy: Omega Factor* video game, where the whistle is given to Astro Boy without his direct involvement.