Yu☆Gi☆Oh! Duel Monsters GX

Synopsis

Atticus has lost his memory and Alexis goes to the forbidden dorm where he disappeared originally. She gets sucked into a mysterious dark realm, where she duels Titan, who has disappeared after his duel with Jaden and has now become the sixth Shadow Rider. His darkness monsters where overcoming her, but Alexis pulled through and defeated him, and was sent back to the Shadow Realm. Afterwards, Atticus remembered who asked him to come to the dorm in the first place, Professor Banner.

(Source: Wikipedia)