Yu-Gi-Oh! Zexal

Yuuma Tsukumo's goal is to become the greatest duelist, but his lack of skill makes this dream seem unattainable. His life changes when Ryouga "Shark" Kamishiro splits a key given by his father, inadvertently setting Yuuma on a path to achieving his goal. Yuuma's inexperience makes him lose a duel to Shark, but the key repairs itself and an otherworldly being named Astral appears before him. Astral explains that Yuuma's memories have been divided into 100 "Number Cards," which he must retrieve to restore his normal life.