Taishiro Toyomitsu, known as the BMI Hero Fat Gum, is a professional hero who operates an agency, primarily in the Kansai region of Japan. He is recognizable by his very large, rotund stature, which is complemented by a hero costume consisting of a zippered hood, gloves, and metallic boots. He speaks with a Kansai dialect, which adds to his approachable and friendly demeanor.

Fat Gum is characterized by a cheerful, friendly, and dependable personality, often described as an easygoing and kind mentor figure. He is almost always eating to fuel his Quirk and appears to be constantly hungry. Despite his relaxed and often playful attitude, he becomes serious and focused when a situation demands it. Having spent time working with the police, he is very knowledgeable about the criminal underworld, particularly regarding illegal Quirk-enhancing drugs. He is also shown to be an emotional and impulsive individual who strongly dislikes waiting and is driven to act immediately to protect lives.

No reliable direct information exists regarding any broader motivations beyond his professional duties as a hero to capture villains and protect civilians. His role in the story is primarily that of a supporting professional hero and mentor. He is first introduced as the supervisor for Tamaki Amajiki's work study and subsequently accepts Eijiro Kirishima as another intern at his agency. He plays a significant role in the Shie Hassaikai raid, where he fights alongside Kirishima against the villains Rappa and Tengai. Later, his agency is involved in major conflicts, including the battle against the Paranormal Liberation Front and the international crisis with Humarise.

Fat Gum has several key relationships within the hero community. He acts as a mentor to Tamaki Amajiki, recognizing his immense potential while also playfully teasing him, which Amajiki sometimes jokingly refers to as power harassment. He similarly mentors Eijiro Kirishima, seeing great potential in him and commending his bravery and fighting spirit after the battle with Rappa. He also works alongside police detectives, such as Naomasa, and other heroes like Eraser Head and Monika Kaniyashiki on joint operations. His agency later also includes Tetsutetsu Tetsutetsu as an intern.

Fat Gum's character development is partially explored through the spin-off series Vigilantes, which shows his past as a lanky and somewhat awkward teenager. This backstory highlights the significant physical transformation he has undergone to become the hero known for his massive frame. In the present timeline, his development is often demonstrated through his leadership and the trust he places in his young charges, allowing them to face threats while providing support, and acknowledging when he has misjudged their capabilities.

His notable ability comes from his Quirk, Fat Absorption. This Quirk allows his body fat to absorb and trap incoming objects or people, making him adept at capturing villains and shielding himself from attacks like bullets and blades. The Quirk also enables him to absorb the kinetic energy from physical strikes, storing it within his fat. He can then release all this stored energy in a single, powerful offensive counterattack. However, using this stored energy rapidly consumes his body fat. After a significant expenditure, he enters a state known as Low-Fat, where his body becomes lean and muscular but his defensive layers are almost completely depleted, leaving him vulnerable. To maintain his protective fat layer and fuel his Quirk, he must consume massive amounts of food continuously.