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Tear Jerker: Yu Gi Oh GX
Jaiden trapped in the mind of the Supreme King. The happy-go-lucky hero who's reaction to immensely powerful harbingers of doom and darkness have ranged from comedic to inspirational is trapped in a perpetual Despair Event Horizon where he's unaware of the horrible things the Supreme King is using his body to do, and where he can only wallow in self-loathing as he blames himself for his friends' apparent deaths.
The above entry is so traumatic for him that he spends almost the entirety of the final season as a cynical Hurting Hero. He rarely spends time with his friends, almost never smiles, only duels—his passion and most joy-inducing pastime—when doing so is absolutely necessary when it comes to saving the world.