Wonder Masamitsu is a magician with dwarfism who joins the Red Cat Circus, performing Western-style magic. His signature act involves fitting himself into a small bottle, captivating audiences. He takes an immediate interest in Midori, a young orphan girl working at the circus, and makes her his assistant. This relationship evolves into a romantic and sexual attachment despite their significant age difference, with Masamitsu becoming her protector against the abusive circus performers.
Masamitsu displays a complex personality, showing gentleness toward Midori contrasted by sudden violent outbursts. Mockery about his stature provokes extreme rage. His possessiveness over Midori manifests as controlling behavior; he sabotages her opportunity to leave the circus for an acting career by destroying a filmmaker's contact details and physically assaults her when she attempts to rebuild this connection. His magical abilities appear supernatural, enabling him to transform himself and Midori into dragonflies, conjure idyllic visions of her deceased parents, and enlarge Midori to intimidate her abusers.
After killing fellow performer Muchisute by suffocating him with sand during a confrontation, Masamitsu's instability escalates. Audience taunts about his height during a performance trigger a magical retaliation that grotesquely deforms the entire crowd, leading to police intervention and the circus's shutdown. Fleeing with Midori when the circus disbands, Masamitsu meets different fates: in the manga and 1992 anime, a thief stabs him to death while he buys food. In the 2016 live-action film, he dies from exhaustion after transferring his magical powers to Midori. Following his death, Midori hallucinates Masamitsu laughing at her alongside other circus members before they vanish, leaving her isolated in a void.
Psychological interpretations within the narrative frame him as an INFP personality type with Enneagram Type 7 traits, highlighting his rich inner emotional life, creativity, and tendency to avoid discomfort through novelty-seeking behavior.