MX appearance:
When disguised, MX looks exactly like Mario does in the original Super Mario Bros. game, with dark tan skin, a red hat and overalls, and a brown-green shirt with matching mustache and shoes.
In his default appearance, MX's skin is pale white, his mustache, shirt, hair, shoes and eyes are black, and his hat and overalls are a darker shade of red than the actual Mario's. He's also much bigger and fatter than Mario and all of his "clothes" are actually part of his body, explaining why they also change in appearance and size when he shapeshifts.
MX's true form is that of a black, flame-like, eyeless spirit with several glowing white teeth.
MX personality:
MX is an exceptionally ruthless and demented entity who will stop at absolutely nothing to claim as many souls as possible to integrate into the game and make his playmates for all eternity. Having existed since the inception of the universe, all of his pent up murderous sadism has waited billions of years to be unleashed on unsuspecting humans for his own enjoyment. He appears to have zero remorse for his actions, dismissing the innocence of his child victims, claiming that it won't get them far, confirming that he doesn't at all care about the age of his victims as long as he gets to torture and murder them.
He is also highly intelligent and manipulative, having convinced Lucas that he was trustworthy in order to gain the advantage over the child and ultimately lead to his death. He's also smart enough to ensure that his victims have no means of fighting back against him, as demonstrated by him removing all the powerups from World 1-1 and possibly the rest of the game, which may hint at a cowardly side to him if he's willing to remove anything from the vanilla game that can be used against him.
In spite of his wicked nature, MX's general demeanor is that of a relaxed, calm and collected entity that can genuinely be polite to people when he wants to be and can strike up genuine friendships with people, especially those with common interests as him such as Lord X, likely from taking some of Mario's personality traits, mainly his friendly nature.
MX origin story: MX's background is entirely unknown, but aside from being an ancient spirit, he is said to have existed since the beginning of time and was created alongside the universe. In late-1985, MX possessed a copy of Super Mario Bros. that ended up in the possession of an eight-year-old boy named Lucas after being purchased by his uncle, Mark for his birthday. A week after beating the game, Lucas noticed that it had begun acting strangely, and eventually made contact with MX, who encouraged Lucas to "go to sleep" and that "the lights wouldn't hurt". Soon after the two made contact, Lucas would be found dead of a seizure and MX claimed his soul, absorbing it into the game and placing it in the body of Luigi.
After capturing Lucas' soul, MX altered the game so that Lucas would have unlimited lives, allowing him to hunt down the poor boy and kill him over and over again.
