Gladys eventually showed up, not in December Bride, but in the spinoff Pete and Gladys.
From roughly the same time period, Angela Valentine, Beaver Cleaver’s six-toed classmate, who several times brought the class to a halt by throwing up.
On the other hand, you have characters like Chuck Cunningham on Happy Days that somehow ceased to exist by the end of the show’s run (Mrs. C’s comment about her two wonderful children.
He didn’t cease to exist! He just got shipped off to prison! He wasn’t such a wonderful child.
On Hill Street Blues Detective Mick Belker used to have a lot of phone conversations with his mother. I don’t think we ever saw her on camera.
I’m pretty sure we never saw her. She died in one of the last episodes, which finally allowed Al to be free of her and led to him marrying his girlfriend. The funeral featured a huge coffin, with about a dozen pallbearers 
BTW, we never saw Tim Taylor’s boss, Mr. Binford, either.
I don’t think we ever saw “Scary Naked Guy” who lived in an apartment with windows facing Monica & Rachel’s apartment in Friends. He is referenced many times, and Ross even got naked with him once.
We sometimes hear (Waa-waawaa. Wa-waa) but don’t see any adults in the Peanuts cartoons.
In Mork & Mindy, Mork ended each episode with a report to his boss, Orson, whom we hear but don’t see.
In the episode that Ross got naked with him, we saw “parts” of him.
A somewhat comprehensive list is on wiki .
Vera was shown. She either had an affair or Norm thought she was having one. But she did appear on the show.
Is Mr Kim on Gilmore Girls still supposed to exist, or are they just forgetting about early references to ‘parents’?
The only time I remember her appearing was when they had a Thanksgiving episode where a food fight broke out, and Vera’s only appearance has her covered in whipped cream. I think she was played by George Wendt’s real wife.
Bob Kelso’s wife on Scrubs.
The Senior Partners of Wolfram & Hart on Angel.
(OK, one of them, once. But he was taking the shape of a lesser demon, and was wearing a hood, so we never actually SAW him.)
The parents of every Disney child character. They all have uncles but never seem to have a mom or a dad
Another unseen MAS*H character was Sparky.
All the grownups on any Peanuts special
More than heard, Carlton was actually seen (except for his head) while carrying something out of the apartment. I’ve always wondered if it was really Lorenzo Music we saw.
As for MAS*H, what I wanna know is who was the voice on the loudspeaker supposed to be? The mike was in the CO’s tent, but it wasn’t Blake, Potter, Radar, or Klinger. It was always the samer, unidentified voice, and in no episode did we see someone who had a voice like that. So who was it?
Did we ever see “Mad Uncle Eimar” from the original “The Addams Family” TV show?
I always thought the voice annoucing incoming wounded was Radar, who in the movie and at the beginning of the series was more than a little psychic, and knew when the choppers were coming (or the phone would ring) before anyone else.
StG
But it doesn’t sound like Radar, and after Radar left, the same voice continued over the Speaker. Plus, you heard it even when you saw Radar outside. The voice was Someone Else. The Magic Voice of the 2077th.
Joanie’s best friend Jenny Piccalo wasn’t seen on Happy Days until Cathy Silvers played her in Season 8.
–From IMDB http://akas.imdb.com/title/tt0068098/faq
Two characters we originally weren’t going to see when the shows were developed, but did, were the President in “The West Wing” and Alan Brady in “The Dick Van Dyke Show”.