This is a list of characters in the series who were mentioned, but never appeared physically.
Ernie and Bert
In Evil Prevails?.
In Evil Prevails?, when Bowser asks the Oogie Boogie Man why he sings and rhymes all the time, his reply is "well you see, it's kind of a curse, I have to sing all day and watch Ernie and Bert".
In Meet Jack Skellington and Mario's Rock Band.
In Meet Jack Skellington, after Jack was rescued by Mario and Luigi, Jack thought that Bowser also kidnapped his girlfriend Sally. However, it turns out to be Toad.
Sally makes a cameo in Mario's Rock Band when Jack demonstrates his singing abilities to Mario and Luigi by showing them a scene from The Nightmare Before Christmas.
Winnie the Pooh
In Revenge of Kermit, Luigi's Bad Luck, and The Blue Terror Bloopers.
In Revenge of Kermit, Kermit ties up Mario and Luigi and makes them watch Winnie the Pooh's Christmas, but they escape.
In Luigi's Bad Luck, Mario uses a Winnie the Pooh on a stick so Luigi can get over his fear of Winnie the Pooh.
In The Blue Terror Bloopers, Mickey uses the Winnie the Pooh on a stick that Mario used in Luigi's Bad Luck to scare off Gonzo.