Janus, born Takeru Hojo, is a world karate champion recruited by Sir Miles Nevers to defend Earth against an alien menace traced to ancient astronauts. His martial arts mastery is demonstrated in a flashback where he karate-chops a bull. Initially a disgruntled antihero, he refuses the first request to pilot combat machinery, characterized by distinctive unkempt hair and casual denim attire contrasting with the Nevers organization's formal uniforms.

He operates the male component of the Magnos robot system, designated "Puraiza" or "Magnetman Plus." This unit combines with Lady Esther Nevers' "Magnetman Minus" through a physical synchronization sequence where they leap from their robots, whirl face-to-face mid-air, and merge into a rectangular yellow module forming Magnos' belt buckle. Janus employs his agility, reflexes, and karate techniques during battles against antagonist-created monsters.

His development focuses on shifting from a solitary fighter to a reluctant team member. After witnessing the inadequacy of Nevers' forces during an alien attack, he intervenes to demonstrate superior combat effectiveness, leading to his eventual acceptance of the pilot role and adoption of a specialized jumpsuit. His dynamic with Esther involves initial friction, adhering to the recurring pattern where the male and female leads clash before cooperating.