Mad Dog, known as Mad Dog Inukai in the Japanese version, is Zane Truesdale's opponent in Zane's first Underground Duel.
In the Japanese version, "Mad Dog" is his nickname, and translated literally, his given name means "dog owner". In the English dub, his voice is an imitation/parody of Mike Tyson; excluding his growls, his voice is very high-pitched and childish, contrasting with his physical appearance.
Mad Dog was an Underground Duelist. He defeated twenty Duelists in a row and earned the title, "Gatekeeper of Hell".
In an Underground Dueling bar, Mad Dog faced an arranged opponent, Zane Truesdale, who had been relegated from the Pro League, following a losing streak. As the Duel began, Mad Dog taunted Zane, by calling him "Pro League Dog". Both players had electrodes strapped to them, so that they would receive high voltage shocks each time they took damage. Mad Dog was aware of the cards Zane used and was able to counter them. He quickly lowered Zane's Life Points to 300 and raised his own to 15000. Zane became frustrated by his losing streak and abandoned his respectful way of Dueling, in favor of more ruthless strategies. Zane Summoned "Chimeratech Overdragon" at 4800 ATK and attacked Mad Dog repeatedly, dropping his Life Points to 0, knocking him out and blasting open the cage.
Mad Dog plays a "Slime" Deck which focuses on various disrupting strategies meant to impede his opponents' ability to activate cards in addition to cards that gradually inflict effect damage under such circumstances such as "Last Machine Acid Virus". He also includes a number of cards that increase his draw power.