He's an ancient spirit-like demonic who inhabited a copy of Super Mario Bros. and disguised himself as a monstrous eyeless version of Mario himself in order to lure unsuspecting players/victims into the game and play with for all eternity.

MX is a particularly ruthless and demented entity who will go to any length to claim as many souls as possible in order to integrate them into the game and make them his playmates for all eternity.

Yet this version goes "AUGHHHH!!!" most of the time.