FightingMario54321, whose username is often abbreviated to FM54321 or FM, is a YouTuber, as well as one of SuperMarioGlitchy4's close friends. He used to be a Super Mario 64 machinimist and blooper maker, but he decided to quit in March of 2015.
His debut in SMG4's bloopers was SMG4 VS. SMG3, inwhich he beats up SuperMarioGlitchy3. 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.
His name sounds similar to MarioMario54321's, and is probably inspired by it.
According to the video "Two Great Friends!", he was formerly a cop (though he really was a police officer in real life), but he got kicked out of this job (only in SMG4's timeline). However he later retrieved it, as seen in the blooper "Can the Villager come out to play?".