FightingMario54321, whose username is often abbreviated to FM54321 or FM, is a YouTuber, as well as one of SMG4's close friends. He used to be a Super Mario 64 machinimist and blooper maker, but he decided to quit in March 2015. His younger brother now currently runs the channel.
His debut in SMG4's bloopers was Super Mario 64 Bloopers: SMG4 VS SMG3, in which he beats up SMG3. Since then he has appeared in several other videos of SMG4's. Good at fighting, – hence his pseudonym – FightingMario54321 is somewhat the tough guy, and acts as one of the few sane characters.
While FM's mainly red CC is somewhat similar to that of the game's default, Mario, there are several chromatic differences, most notably in his hair color, overalls, and gloves.
Red cap, eyes, arms, and shirt
Purple gloves (which sometimes appear to be magenta due to green screen issues)
His name sounds similar to MM54321's, and has been confirmed to inspired by it. The "Fighting" actually came from his admiration of the fighting scenes a retired YouTube user named "LuigiGame2" used to make.