Mamoru Murakami is the son of journalist Atsushi Murakami and Tomoko Murakami. When his father attempts to expose the alien Goa's invasion plans for Earth, the family becomes targets. Goa demonstrates his power by transporting them to a prehistoric jungle, where they witness a giant dinosaur's destruction, thrusting Mamoru into the conflict against Goa's forces.

Following this encounter, Mamoru meets Earth, the planetary creator dwelling beneath a volcanic island. Earth grants Mamoru a specialized whistle to summon protectors: one blow summons Gam, two summon Mol, and three summon Magma. This whistle becomes central to Mamoru's role in combating threats.

Magma develops an emotional connection to Mamoru, reflecting his desire for a child with his wife Mol. In response, Earth creates Gam, a robotic duplicate modeled directly after Mamoru. Gam, wearing a white helmet with antennae, joins Magma and Mol as defenders, forging a unique bond with his synthetic counterpart Mamoru.

Mamoru actively investigates Goa's schemes alongside Gam, acting before threats escalate to require summoning Magma or Mol. His actions frequently place him at the center of conflicts involving Goa's monsters and Lugo Men operatives. A significant story arc involves the kidnapping of his mother, Tomoko, by these operatives, intensifying his personal stakes across multiple episodes.

In the 1993 OVA adaptation, Mamoru retains his core role: his family is targeted by Goa, he receives the summoning whistle from Earth, and he collaborates with Magma, Mol, and Gam against recurring threats. The adaptation maintains his investigative activities and reliance on the whistle during crises.

The English adaptation renames Mamoru as Mikko Mura but preserves his familial relationships, his receipt of the whistle, and his function in summoning the protectors (renamed Goldar, Silvar, and Gam). His character continues to engage directly with the primary antagonist, Rodak (Goa), and his forces.