Yubel

Yubel is the primary antagonist of the third season of Yu-Gi-Oh! GX. An evil Duel Monster whose card was in Judai's possession when he was a little kid, Yubel is deranged and manipulative, mainly aligning themself with others to accomplish their goals. They first act through Professor Cobra and Martin, being supposedly defeated by Judai and his new ally, Johan, after showing themself. However, behind the scenes they orchestrate the events that led to Judai's descent into darkness and later returns as the final antagonist of the season, using the power of the "Super Polymerization" card to unite all twelve Duel Spirit dimensions together and reign over them with Judai at their side. At the end of the season 3, we learn that Yubel was the guardian of Judai, who promised to love them forever.