He only got that job by kissing The Big Giant Butt.
What about Eddy’s son on Absolutely Fabulous? Did he ever appear?
Did Sheridan of Keeping Up Appearances ever appear?
I know Hyacinth’s sister Violet was referenced to a lot but didn’t appear till the last season
Would the “little red-haired girl” from Peanuts count? She was never seen in the strip, but did appear in a couple of animated TV specials.
Carlton, your Doorman from Rhoda. Voiced by Lorenzo Music, his arm was seen and the back of his head was seen. The show he was on was a spin-off from the Mary Tyler Moore show and they attempted a third spin-off, as an animated series.
As in the posh British pronounciation of “schedule.”
<fingers in my ears> LA LA LA!! I CAN’T HEAR YOU!!! That NEVER HAPPENED! And her name CERTAINLY isn’t Heather!
True enough. From the Wikipedia page I linked to:
Schulz did not consider these animated appearances to be canonical (although he wrote the screenplay himself). Officially, she remains unseen and unnamed.
And from the page it cites as a reference:
Schulz never had full control over what happened in the TV
specials, and he made it very clear that events therein should
not be regarded as “gospel” for his newspaper strip. Thus, while
it’s true the little red-haired girl popped up in the TV special “It’s
Your First Kiss, Charlie Brown” (and therefore in the picture-book
adapted from that show), Schulz did not regard her as
the actual little red-haired girl…just as Charlie Brown’s having
successfully place-kicked a football in another TV special
should not be regarded as a comparative truth in the strip.No, the “actual” little red-haired girl, like the Head Beagle, or
Linus’ Miss Othmar, is a character who never was drawn.
This way, we can all imagine her to be whatever we’d like…safe
in the knowledge that there is no visualization of the character
which is better, or different, than any other.
Though I CAN accept the holiday themed (notable exception: New Year’s Eve) specials as “canon”.

Jenny Piccolo on Happy Days.
And Chuck Cunningham…

Did we ever see Phyllis’ husband Lars on the Mary Tyler Moore show?
There were references/phone calls to Mary’s parents…I believe they showed up for a visit sometime after the first couple of seasons.
I’ve heard that LRHG did appear in a canonical strip drawn by Schultz himself, albeit in silhouette, and non-speaking.

Peter_Morris:
shooled?
As in the posh British pronounciation of “schedule.”
I’m British, and I’ve never heard anyone pronounce schedule as shool.

Annie-Xmas:
Have you ever seen Clarissa Flockhart?
Extremely low weight was only one of Maris’s physical abnormalities though. She had multiple plastic surgeries, couldn’t produce saliva, etc.
I always pictured Juliet Landau as Maris.

On Fringe, William Bell isn’t revealed to be Leonard Nimoy until the end of the second season.
1st season. I’ve only seen the 1st season and he was in the last episode.

What about Eddy’s son on Absolutely Fabulous? Did he ever appear?
He appeared in a follow-up special to the series. (5:40 in this.) He was basically a gay male version of Saffron- very “straight” laced and serious and living in NYC. He and Edina reunite but can’t stand each other, but she absolutely adores his queeny partner and vice versa.
I don’t think Sheridan was ever seen.
Friend’s Ugly Naked Guy. Talked about him a lot, but he only made two appearances–once when they were poking him, and once talking to Ross.
Sir Roysten on Drop the Dead Donkey, very last ep IIRC.
Number One in The Prisoner, of course. Showed up (or did he?) in the very last episode.

Columbo often referred to his wife, but she was never seen until the spinoff Mrs. Columbo, where she was played by a post-Ryan’s Hope and pre-Voyager Kate Mulgrew.
Pshaw, there is no Mrs Columbo.
On the daytime series “Dark Shadows”, after the first season, virtually every main character had been mentioned for at least some time before they were seen. Sometimes for long periods of time before they were seen… Barnabus Collins, Quentin Collins, Angelique, etc.