There’s a long tradition of TV characters who were often referenced and sometimes heard but never seen: Carlton the doorman on Rhoda, Norm’s wife Vera on Cheers, Maris on Frasier, etc… But there are a few who were often referenced and never seen until later in the series, when suddenly they pop up.
Two I can think of offhand, but I know there are others:
Grace Under Fire- Grace’s ex-husband Jimmy, described as a Jeff Foxworthy-sense redneck and all around idiot with a drinking problem, was unseen until suddenly the character becomes a regular played by Geoff Pierson in the third season.
The Nanny- Fran’s father, Morty, was frequently mentioned but never seen- at least not facially or clearly- in the first several seasons. You learn that he’s bald/wears a toupee but has lots of body hair and is probably obese- he sounded like a great role for Barney Martin (the actor who played Seinfeld’s dad) or maybe a younger version of ubiquitous Jewish character actor Lou Jacobi. In the later seasons he had speaking roles and was seen head on and played by singer Steve Lawrence.
Any others who weren’t seen until such time as they were?
Interestingly, Maris from Fraiser actually was supposed to be a recurring character (they didn’t want to duplicate the bit with Vera from Cheers), but by the time they got around to considering how to introduce her, the various jokes had made her too bizarre a creature to cast with an actual human.
On the soap Another World, whenever Rachel Frame went to visit her mother, the writers had her son Jaime get out of the scene by sending him over to play with Joey Perini. Finally, head writer Harding LeMay decided to introduce the whole Perini family. The role of Joey was played by a very young Ray Liotta.
David Hyde Pierce wanted Valerie Mahaffey, probably best known as Eve on Northern Exposure and who had played his super neurotic wife on the short lived Powers That Be, but as mentioned by the time it got around to it even the very thin Vallerey was too heavy and not high strung enough.
In The Guns of Will Sonnett Walter Brennan was the title character who rode with his grandson (Dack Rambo) trying to find his son/his grandson’s father Jim Sonnett, who was never seen. Jim was brought on in the final season for 13 episodes.
In the 50’s sitcom December Bride Pete Porter would complain about his never-seen wife, Gladys. At the very end of the series Gladys suddenly appeared, just in time to be spun off into the sequel Pete and Gladys.
Don’t know how much this counts, but Home Improvement had a really irritating schtick about the next door neighbor: we would only see him from his eyes-upward as he talked across the fence to Tim Allen.
Only in the final episode. What I never understood about that gimmick was that they used an actor, Earl Hindman, who’d had a long career in movies and TV- he was on soap operas for many years- so his face was well known and easily found in any number of videos and likely to be on television at any time. An unknown would perhaps have been more enigmatic.
Extremely low weight was only one of Maris’s physical abnormalities though. She had multiple plastic surgeries, couldn’t produce saliva, etc.
I don’t think it was supposed to be enigmatic, just a joke on the fact that during the first few seasons his only interactions with the rest of the cast was peaking over the back fence, so when they had him appear in other settings, they kept the same half-view of his face.
The early years of “All in the Family” George Jefferson never appeared. They talked about him frequently. Once the B unkers had an a man over for dinner who Lionel introduced as his father but at the end turned out to be his uncle Henry because George wouldn’t set foot in Archie Bunker’s house. At the time the explanation was Norman Lear always wanted Sherman Hensley to play George but he wasn’t willing until later to sign a contract.
On Roseanne, Bonnie Sheridan, played by Bonnie Bramlett of Bonnie & Delaney fame, mentioned her husdand Duke several times in Season 3 He only showed up once, in season 4, played by David Crosby. They did a damn fine duet.