Juudai is your regular Duelist, who loves to play the game. He's a member of the Slifer Red dorm and is usually called a Slifer Slacker. He wants to become the next "King Of Games" and is willing to duel anyone for the title.
Juudai has the ability to see Duel Monster spirits. His spirit partner is Winged Kuribou. In the anime, Juudai received Winged Kuribou from Yugi Moto while in the manga he received it from Kouyou Hibiki.
In the anime, Juudai first plays an "Elemental Hero" deck, then a "Neo-Spacian" deck but now he plays a deck combined with "Elemental Heroes" and "Neo-Spacians." His strategy is based on fusing monsters together. In the manga, he plays an "Elemental Hero" deck, however it differs from the deck in the anime.
His alter ego, The Supreme King is one of the main antagonists of the third arc and season of Yu-Gi-Oh! Duel Monsters GX. He is considered one of the darkest characters in the anime. Specifically, he serves as the main antagonist starting in episode 136 until his defeat in episode 143.
The Supreme King is cold, collected and ruthless, and is willing to sacrifice as many people as he has to in order to complete Super Fusion. He is also very taciturn; he only talks when necessary or to spite his enemies, and when he talks, he speaks in a slightly condescending tone. His demonically golden eyes (and overall visage) show no emotion other than anger and hatred.
While in possession of Juudai's body, he plays an "Evil Hero" Deck, vile incarnations of Juudai's "Elemental Heroes." Like Jaden, The Supreme King's tactics revolve around Fusion Summoning except he uses his own signature card, "Dark Fusion," a warped version of Juudai's traditional "Polymerization." "Dark Fusion" prevents the monster being destroyed by Spell and Trap Cards during the turn it was summoned. The Supreme King also has access to the "Super Polymerization" card, which can use Fusion Material from both sides of the field and can't be negated by Spell and Trap Cards or the effects of Monster Cards. It is able to bring out any kind of high-Level Fusion Monster with dreadful effects in an instant.
Unlike Juudai, whose unorthodox plays depend on chance, The Supreme King is straightforward and his arsenal uses a variety of effects that range from card destruction to immunity from opposing card effects that effectively reflect the supremacy and omnipotence of a monarch. The Supreme King possesses the alternate art version of Elemental Heroes "Avian," "Burstinatrix," "Sparkman" and an unreleased version of "Wildheart."