Koichi Yamano, a seemingly average Japanese junior high student, awakens to a legacy tracing back to Babel—an alien entity stranded on Earth millennia ago, whose abandoned Tower of Babel now recognizes Koichi as its heir. Designated "Babel II," he inherits superhuman intellect, physical prowess, and psychic mastery: telepathy, telekinesis, energy projection, and regenerative capabilities. Three ancient guardians forged by Babel aid him: Rodem, a shapeshifting entity oscillating between panther and human forms; Ropross, a robotic pterodactyl armed with supersonic speed and projectile weaponry; and Poseidon, an aquatic humanoid machine engineered for naval warfare. Together, they oppose Yomi, a rival claimant to Babel’s legacy bent on global conquest through cyborg infiltration and robotic armies, his ambitions fueled by rejection from the Tower’s systems.

Koichi’s ascension draws dangerous attention. Captured by the CIA in *His Name Is 101*, his DNA fuels a project to mass-produce artificial espers. Stripped of his imprisoned guardians, he escapes and dismantles the unstable clones. Decades later in *The Returner*, his slowed aging belies heightened powers—time manipulation, interdimensional thought traversal, energy beams—though each exertion risks bodily collapse or accelerated senescence. He confronts resurgent threats: Yomi’s renewed schemes and U.S. militarized forces targeting his capabilities.

Beyond solo battles, Koichi emerges in crossovers like *Giant Robo* as the shadowy "Big Fire," orchestrating covert operations through a global network. Regardless of era or adversary, he navigates moral complexities—deploying psychic subterfuge, tactical alliances with authorities, and calculated guardian strikes—to shield humanity while safeguarding the Tower, his eternal stronghold against chaos.