Baikinman originated from Baikin Planet, arriving on Earth in an egg that hatched simultaneously with Anpanman's birth. His existence centers on defeating Anpanman and spreading germs, establishing their dynamic as representations of moral dualism. Physically, he resembles an anthropomorphic demonic fly with a black furry body, large teeth, pink eyes, purple lips, antennae, fly-like wings, a zigzag-shaped tail, and a slimy blue tongue. Early depictions presented him wearing a supervillain suit, while later media describes his appearance as naturally coated in mold, revealing a bone-white body when sanitized.

His personality exhibits narcissism, impulsiveness, and grandiosity, frequently using the excessively pompous self-referential pronoun "ore-sama." He behaves loudly and rudely, driven by an obsessive need to sabotage Anpanman, rooted in the belief their shared birthday destined him for this role. Despite self-identifying as "a being who knows no love," he occasionally reveals hidden vulnerability; creator Takashi Yanase suggested Baikinman harbors a subconscious desire for normalcy but feels constrained by his nature. He demonstrates unexpected loyalty and protectiveness toward allies like Dokin-chan and Franken-robokun and exhibits gluttony, often stealing food during schemes. Portrayals later incorporated moderated hygiene habits like handwashing and teeth-brushing to avoid negatively influencing young audiences, though full exposure to soap or water still severely weakens or shrinks him.

Key relationships define his actions. Dokin-chan serves as his primary partner in crime, with Baikinman showing persistent romantic feelings for her despite her preference for Shokupanman. Their dynamic blends complicity in villainy with his attempts to impress her. Franken-robokun, a robot he designed to combat Anpanman, defies expectations through inherent kindness, leading Baikinman to avoid him yet simultaneously exhibit a reluctant fondness. Rollpanna represents a manipulated victim; he infected her with bacteria grass to force attacks on Anpanman, though she consciously avoids triggering this transformation. Horrorman, a skeleton allied with him, adds chaotic companionship without deep loyalty.

Baikinman possesses notable abilities and vulnerabilities. As a self-proclaimed genius inventor, he constructs elaborate robots, vehicles like UFOs, and germ-based weapons. His physiology grants resilience, allowing recovery from repeated defeats, and limited flight via insect wings. His critical weakness involves cleanliness: soap or sanitization removes his protective mold coating, causing physical diminishment and pain. Cultural symbolism links him to bread mold (*Rhizopus stolonifer*), naturally antagonistic to Anpanman's bread composition. Design influences include Beelzebub, the "Lord of the Flies" from Christian demonology.

Character development appears across media formats. While consistently adversarial in the main series, non-canon specials depict scenarios where he and Anpanman coexist peacefully, such as navigating real-world challenges like a shopping mall. In the 2018 film *Shine! Kulun and the Stars of Life*, he hesitantly cooperates to save Anpanman's life and refuses to eat his offered head due to pride. His popularity in Japan influenced shifts in portrayal, including moderated hygiene habits and reduced overtly sadistic acts. His design also inspired disease personifications in other media, notably a Baikinman-like computer virus in Pokémon's banned "Electric Soldier Porygon" episode.