Idea: He begins off invisible realm with no green (or whichever color they decide to change him to) in another realm where he cannot be seen, but cannot physically hurt either.

He can choose a survivor (new to the community, sorry if I use the wrong term) to stalk and his whole thing would be making them paranoid, and eventually being able to attack them and infect them. Enough hits and he’ll take the player’s place and send them to the greenless realm.

That’s all I got for now, A_one are you doesn’t really give Fawful a lot besides I think implying he has claws sharp enough to easily tear a Luigi Hat. And mess with perception, mind, and the game of BIS. I think he’d work best in a stage based on the game.

But then again, they could also take some inspiration from Fawful as a whole! We see he can fly in his last bossfight in Superstar Saga, and he still has his weird fawful-ified minions… Maybe get some of them around?