Harmonica (real name unknown) is the deuteragonist of Sergio Leone's 1968 Spaghetti-western film Once Upon a Time in the West. He is a mysterious gunslinger who wants revenge against Frank for the sadistic torture and murder of his brother.
He was portrayed by the late Charles Bronson.
Vote which fictional character would naturally fit in-universe with a big-named franchise! No time-travel, no interdimensional crap, no "Pooh's Adventures", they would just exist in-universe.