Are you sure you want to know, sweetheart? It's a big spoiler as far as the end of Mother 3 goes... But if you're sure, then... The Masked Man is Claus, Lucas's older twin brother, who was killed by the Mecha Drago that took their mother's life. He was found, reconstructed and reanimated by the Pork Army. Because of this, he's lost all memory of his previous life and no longer recognizes his home or brother. The Masked Man will reveal himself eventually...