Hailing from Baikin Planet, Baikinman arrived on Earth as an egg that hatched simultaneously with Anpanman's birth, forging their intertwined destinies. His core purpose is defeating Anpanman, a mission he frequently proclaims with declarations like "I was born to defeat you, Anpanman!"

Physically, he manifests as an anthropomorphic germ resembling a fly. His form features a black furry body, fly-like wings, light pink eyes, purple lips and nose, two antennae, a zigzag-shaped tail, and a slimy blue tongue. Contact with soap or cleanliness triggers a severe allergic reaction, turning his body entirely white and sometimes shrinking him. This design draws inspiration from Beelzebub, the "Lord of the Flies" in Christian lore, and personifies black bread mold (Rhizopus stolonifer), a natural enemy of bread-based characters like Anpanman.

His personality features narcissism, impulsiveness, and a self-proclaimed genius intellect, often channeled into elaborate mechanical inventions aimed at thwarting Anpanman. Despite his primary antagonist role, he exhibits complexity, showing a reluctant willingness to assist Anpanman in dire situations and a hidden desire for a normal life, per creator Takashi Yanase. He displays high resilience, persistently returning after defeats, and gluttony, frequently attempting to steal food.

Key relationships define his actions. His obsession with Anpanman clashes with Anpanman's consistent friendship attempts, creating a dynamic of mutual yet asymmetrical conflict. He nurses unrequited feelings for Dokin-chan, his frequent partner in schemes, though she prioritizes Shokupanman. He created Franken Robo-kun, a robot intended to electrocute Anpanman, who instead developed a gentle personality and views Baikinman as a father figure, eliciting reluctant affection from Baikinman despite the robot's unintentional electrocutive hugs. He also engineered Rollpanna's blue evil heart by inserting a "Baikin Plant" during her creation, enabling him to influence her against Anpanman, though her loyalty remains inconsistent.

His abilities include piloting a personalized "Baikin UFO" for transportation and combat, inventing complex machines and robots for his schemes, and employing signature catchphrases like "Hahihuheho" and the pun-based "Baibaikin!" (combining "bye-bye" and "baikin") during escapes. His weaknesses extend beyond soap to include any form of cleanliness, which physically debilitates him.

Appearing in nearly every episode, Baikinman maintains his status as the franchise's central antagonist. His influence extends beyond the series, inspiring germ-themed characters like a computer virus in a banned Pokémon episode and Vaccine Man in "One-Punch Man," the latter voiced by the same actor.