MAJOR AF SPOILERS FOR MOTHER 3 (and I guess Super Paper Mario too)

Two """mysterious""" antagonists from the third installment of popular Nintendo RPG franchises who are actually the corrupted alter-ego of the main protagonist's brother (Luigi and Claus) after being brainwashed and controlled by one of the main antagonists of their game (Nastasia/Dimentio and Porky Minch). Due to this, even when faced with their brother and their team of rag-tag group of friends they don't hesitate to fight them, although at the end/near-end of the game they manage to regain their original identities, becoming good guys once again. Both at one point end up in the afterlife (Luigi ending up in the Underwhere thanks to Dimentio and Claus being reunited with his mother in the afterlife after promptly sacrificing himself), usually after losing their old identities, fight against their brother for their quest to obtain, collect, or just get to a special set of artifacts (the eight Pure Hearts and the Seven Needles), putting the whole world and existance at edge as, if things go wrong, it could all come to an end.

I guess they're also both in some way the final boss (Much more directly in Masked Man's case but you do partially fight Luigi/Mr. L when you fight against Super Dimentio)