Miss Bellum from the Powerpuff Girls.
Two from British TV-- Sheridan from Keeping Up Appearances and Victor’s mother-in-law from One Foot In The Grave . Might as well throw in Sheridan’s roomie, Tarquin, as well.
Maris is another favorite. When they ran out of skinny jokes, they gave her an enormous appetite in response to her divorce from Niles and made her the butt of fat jokes.
Yeah, we definitely saw Wilson’s bottom half a time or two. The vast majority of his appearances were over the fence, but he’s shown up at the hardware store and th Taylor’s house, among other places, and the director would have to get pretty creative to hide whatever he could.
We never see Summer’s (soon to be ex-) stepmother, Gloria on The O.C. The picture we get is of a completely drugged-out aging gold digger–Summer’s alway making comments about her taking Valium, Vicodin, any number of prescription psychoactives, et cetera., and she’s been married to a very wealthy plastic surgeon.
Don’t know if she’s my favorite, but I’ve been dying to see her for ages!
The Announcer on Superfriends.
He did a walk-on in Family Guy, though…
For years on All in the Family, Stretch Cunningham existed only in Archie’s stories about working at the loading dock. Then Norman Lear had to ruin it all by bringing him on screen.
The narrator in Rocky and Bullwinkle.
Peter Brady in The Invisible Man
We actually did see “Ugly Naked Guy” on Friends.
Any my favorite would be Dave’s Mom on Newsradio. She was hot. Or so we frequently were told.
-Joe
I was always a fan of Robin Masters from Magnum P.I.. We never saw him…or did we? 
Yep, you saw him in virtually every single episode! Higgins is Robin Masters! He is I say!
The voice on the tape recorder from Mission:Impossible.
Interesting trivia about Bob Johnson
Once. Orson Welles played him. His absence in later seasons may be attributable to Welles’ failing health and death.
Wilson on Home Improvement sometimes had the top of his face show, sometimes the bottom. I’ve seen a clip of the cast taking a bow after a taping, and the actor came out with a script covering his mouth. He dropped it to show he’s entirely normal looking, though.
The radio announcer on Gilligan’s Island - always the same coconut-juice-powered voice on whatever station the castaways tuned in.
Randy Beaman on Animaniacs. That was NOT the kid who ran out of the house to tell what Randy Beaman just did.
In voice only and, in the context of the episode, we are not even sure if it was Robin Masters.
Exceot that one-shot special was animated, so, technically, we only saw a drawing of him.
Damn you all!!! Not only was I not able to mention Carleton… I wasn’t even able to say that the Carleton show was animated and therefore not actually him. I hate you and your vast fountains of knowledge.
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Has that Carleton show ever been rebroadcast since the '70’s? Is is available at all?
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Back to the OP… does Jan Brady’s imaginary boyfriend, George Glass, count?
Thing on The Addams Family was never seen, except for his hand of course. 
We saw him once in silouette in a dark room…or perhaps he was an actor hired by the real RM, Higgins, to play him!
I don’t know about being rebroadcast but it’s certainly not on video.
Just like Garry Marshall did on Happy Days when he brought on Cathy Silvers as Joanie’s often mentioned, but never seen, friend Jenny Piccolo.
I’ve gotta go with “Juanita up at the diner” from the Andy Griffith Show. Barney Fife’s flirtatious phone conversations with her were comedy gold.
The show’s other invisible character, Sarah the telephone operator, wasn’t as interesting, though her apparent eavesdropping was good for some chuckles.