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 herself with others to accomplish her goals.

[Spoiler] She first acts through Professor Viper and Marcel, being supposedly defeated by Judai and his new ally, Jesse, after showing herself. However, behind the scenes she orchestrates 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 her side.

At the end of the season 3, we learn that Yubel was the guardian of Judai, who promised to love her forever. [/Spoiler]

[Spoiler] Many fans argue the topic of her gender, although the majority have concluded that Yubel is a woman due to her original voice, her love for Judai and many other factors. [/Spoiler]