Abaku Onifuda leads the Onifuda Overlord Coalition and hosts the "King of Duematch" tournament, where he reigns as the top player. His goal is to target powerful duelists to seize their prized possessions and qualities, his motives shrouded in mystery. He operates with his secretary, Connie, who harbors a romantic attraction to him, and his subordinates Megane and Zancook. After deeming Megane incompetent, Abaku disposes of him and later employs three agents—Kouki, Chouki, and Saiki—to antagonize Joe's party. In the manga, Chouki and Kouki were once brainwashed tournament contestants, while in the anime, all three begin as his agents.

Abaku carries himself with nobility yet exhibits a ruthless streak, growing intense and confrontational when facing strong opponents. During heated battles, his face contorts into unsettling expressions. He identifies powerful duelists by sight alone and senses when opponents devise strategies, allowing him to formulate countermeasures. While he may grant second chances to those he deems worthy, he eliminates incompetent subordinates without hesitation.

In the narrative, he kidnaps Momo Uraraka to compel Daichourou to join his ranks. He duels Kira with ease but offers consolation points for his worthiness. After Daichourou defects upon Momo's rescue by Joe, Abaku confronts Joe over the hostage situation. He lends his trump card to Megane for a duel against Joe, but Megane fails due to poor synergy with the card, resulting in his disposal. Abaku replaces him with three black-hooded duelists.

As a host of world tournaments, he shows interest in recruiting strong duelists like Zero Jr., though unsuccessfully. He duels Joe, revealing his motive to hunt duelists who possess King Master cards. During their match, he employs a Darkness and Fire civilization deck featuring Demonio and Onifuda Kingdom races with Onitime abilities. Key cards include Gokensai, Oni of "Outrage"; Fuujinten, Oni of "Gale"; Jaouga, Oniga Great King; and Hyakki no Jaoumon. He strategically shatters Joe's shields and overwhelms him with high-rarity cards but does not eliminate him due to the point-based ranking system.

Later, he forms a tag team with Connie and Hyde. His deck evolves to include Darkness and Fire civilizations based on Demonio and Oni RexStars races. A flashback reveals his first deck was a Darkness Fire Hellborof deck built with Himiko. In the manga, his first deck is based on Onifuda Kingdom, and his second on Oni RexStars.

Abaku returns in subsequent events as a figure enchanted by demons, now transformed into a demon himself, his goals growing more enigmatic and central to the story's climax.