Mario

Mario never appears in the canon, but is mentioned as existing in the same universe and living on Mushroom Planet in Meta Knight Story. He is the Mario from Super Mario fame.

He makes one appearance in the hidden non-canon story, Teleportation Part 1. A warp opens somewhere in the Mushroom Kingdom and brings him, Luigi, Yoshi and Bowser over to Pop Star. He introduces himself, and then the unfinished story ends on a to be continued.