Interesting story on that: Peg’s mother was scheduled to become a semi-regular played by Divine , who died just before shooting was to begin. Rather than replace the irreplaceable Mr. Milstead, they left her as an unseen character (though Tim Conway played her husband; once there was a scene in which she was sitting on the opposite end of the sofa from her husband and he was up in the air and that was as close as it ever came.)
While never seen in her husband’s show, she eventually got a show of her own, staring Kate Mulgrew. Actually, it wasn’t half bad…but it didn’t last.
Before the spinoff, Doris Belack played the role of Bernice in one episode, and later Florence Stanley played the part.
Also, there was an early episode of Leave it to Beaver where Larry is seen talking to his father. The man has no line, so therefore no credit.
Yeah, we do. Up until I saw that episode, I had always thought of Duke as one of those unseen characters. I don’t remember seeing him again afterward, but he is there in the bar episode.
Hey…my sig line is suddenly really appropriate! 
Pete’s wife, Gladys, in December Bride, though she was shown in the spin-off, Pete and Gladys. BTW, Pete was played by Harry Morgan, which means Morgan had invisible wives on two different shows (the other being MAS*H), and both showed up on spin-off (AfterMAS*H).
Mr. Terhorst, Bob Newhart’s boss in Bob.
Another MWC gimmick: Gary, the owner of Gary’s Shoe Store where Al worked, was never really seen in the first few seasons: one time Gary did show up to praise Al, but it turned out to be an actor Al had hired for the occasion. Gary must have sold the store to somebody who also went by Gary but was a woman, played very formidably by Janet Carroll. Gary (woman) even had an affair with Bud!
The Control Room in–dang, memory just ain’t what it used to be…that Canadian-produced SF show about the generation ship that was headed into a sun–anyway, as someone said, once they show the invisible character, it’s all over. When Harlan visited the set of the series, they were building the Control Room and he said “What are you doing? If they find the Control Room they can turn the ship and the series is over!” Typical TV series lack of thinking.
That would be The Starlost.
Chef on Enterprise is a character talked about but never seen. There is one episode where the crew had to sleep in the nacelles (sic?) where Chef’s legs were seen.
Why is this, I do not know.
How about Edina’s son Serge on Absolutely Fabulous - or was he eventually shown?
God in all those Hebrew National commercials