Biomon is an Ultimate-grade Appmon who serves as a member of the Ultimate Four, a group of powerful beings acting under the command of the malevolent artificial intelligence Leviathan. His designated application is Life, and he is widely recognized as a renowned physician of phenomenal skill, with a reputation that there is no patient he cannot cure. For Biomon, who possesses an all-encompassing knowledge of living organisms, even complex surgical procedures are considered trivial.

In terms of personality, Biomon embodies the archetype of a mad doctor. He approaches his role with a professional demeanor but harbors a cruel and sadistic nature, especially when dealing with his subjects. He is confident, often bordering on arrogant, and takes a disturbing amount of pride in his surgical capabilities. This is demonstrated when he gleefully operates on his targets, showing a clear disregard for their well-being and a preference for inflicting torment. His curiosity drives him to perform extreme and unethical experiments, treating living data as mere material for his work.

Biomon’s primary motivation is to serve Leviathan’s overarching plan to reshape the world. He is initially tasked with locating the elusive Bootmon and eliminating the App Drivers who oppose Leviathan. However, his personal motivation is deeply tied to his identity as a surgeon and his connection to one of the main characters. He is driven to complete his most significant "operation" and seems to take a particular, almost obsessive, interest in the App Driver Rei Katsura.

Within the story, Biomon is first introduced alongside the other members of the Ultimate Four as a major antagonist. He is the one responsible for a pivotal and cruel act: the surgical transformation of Rei’s younger brother, Hajime, into an Appmon. He confronts the protagonists directly, spreading a special AR field known as the Bio Royal Palace, which he has customized for his surgical procedures, and easily overpowers Rei’s partner, Hackmon. During their rematch, he captures Rei and prepares to operate on him, only to be defeated when Rei and Hackmon work together. His defeat reveals the shocking truth that Biomon himself is the very Appmon that Hajime became, meaning Rei was forced to fight his own brother. Biomon’s chip is retrieved but remains inactive until the final arc, where it fuses with Revivemon (Hackmon's Ultimate form) to create the God-grade Appmon, Hadesmon, the king of the underworld who presides over destruction and regeneration.

Key relationships define Biomon’s role. He is a loyal subordinate to Leviathan and an ally to the other members of the Ultimate Four, such as Charismon, though he often acts independently in his "medical" pursuits. His most significant relationship is with Rei and Hajime. As the one who transformed Hajime, he is the direct object of Rei's vengeance and anguish, while also being unknowingly the brother Rei seeks to save. This creates a tragic and complex dynamic, as Rei must ultimately accept Biomon as Hajime to unlock their combined potential as Hadesmon.

Biomon possesses several notable abilities that make him a formidable opponent. His signature techniques include Needle Blaze and Scalpel Wizard (also known as Dokter Mes or Nadel Blaze), which he uses to attack and dissect his foes. He can use an ability called Emergency Room, which fires a stabilizing beam that paralyzes his target, freezing their condition to prepare them for surgery. He is a master of data manipulation, able to resurrect defeated Appmon and, most horrifically, alter the very nature of a being's existence, as he did with Hajime. He also has the power to create a specialized AR field, the Bio Royal Palace, which serves as his personal operating theater and grants him an advantage in combat.