Randy Carter, born Koichi Hoshi, operates under the codename P77 as a secret agent for the international law enforcement agency Phoenix. His official cover is a manga artist, masking his true activities. He joined Phoenix seeking vengeance for a friend, the previous P77, killed by the adversarial group Interspy.

He relies primarily on martial arts and specialized gadgets over firearms. His modified wristwatch incorporates a small hammer and chain, radar, and a flashlight capable of emitting blinding beams. His pipe conceals smoke-emitting cartridges for diversions and escapes. Randy is also a master of disguise, facilitating covert missions. Phoenix policy stresses minimizing violence, permitting lethal force only as a last resort.

Randy maintains a close bond with his younger brother, Kenny Carter (Shinichi Hoshi), who knows the extraterrestrial mission of the Wonder Three. This connection leads to occasional collaboration, with Randy using Phoenix resources to aid Kenny and the alien agents during crises. Conversely, the Wonder Three have intervened to protect or rescue Randy in high-risk situations. His main adversary is Interspy, though he confronts other global threats assigned by Phoenix leadership.

He reports to the enigmatic head of Phoenix, known only as M, receiving assignments and intelligence; M's identity remains concealed, typically shown only from behind. Randy's parents run a small hotel; his mother is characterized as domineering, his father as comparatively passive. They remain unaware of his clandestine activities.

Throughout the series, he consistently balances global security duties with familial loyalty as a dedicated operative, adhering to his established background and responsibilities.