Also… Howdy Doody. The familiar image is not the first version of the lovable string puppet. After a contract dispute, the original puppeteer quit and took Howdy with him. While the producers were getting a new Howdy made, Buffalo Bob made like Howdy was stuck in a drawer for a few weeks, until the second, more popular version was ready to debut.
Nina’s friend Binnie was shown just once on Just Shoot Me, but not her face, so she would go in category 1. The epsisode where they showed her had Elliot and Finch telling Nina they thought she was made up, since no one ever saw her.
Although they didn’t show adults in any of the TV specials or the series, they did in at least one of the feature-length movies (I think the one where they go to France). Also, The Little Red-Haired Girl was onscreen as far back as It’s Your First Kiss, Charlie Brown.
How about Karen’s husband on Will & Grace, Stanley is it?
AFAIK, we haven’t seen him, but I stopped watching a couple years ago. Seems like a common occurence in Burrow’s TV shows.
In addition, there was also Lomez.
Well, I think I will post the fact that I have seen the LRG in the comic strips, and ::Crash::
WTF! I have seen her, (Peanuts: The Art of Charles M. Schulz and this video box harlot looks nothing like her. No wonder. According to the above link, her design was done with no input from Schultz.
I saw an episode in which Will, Grace, Karen & co. went to Stan’s funeral and will-reading. It was a rerun, so he’s been dead for at least a year or two. Stan had kids though, and (presumably) Karen got custody of them. What happened to them wasn’t mentioned. They, likewise, have never been seen.
Not sure about these two, but -
On “That 70s Show”: the guys frequently mention the name Pam Macy, implying that she’s the trampy, easy girl in their school. I don’t recall ever seeing her though.
On “Frasier”: Martin has a friend “Duke” who he frequently talks about, but I can’t remember ever seeing.
Here’s an old one: Mr. Bloodhound, from “The Bloodhound Gang” shorts on 3-2-1-Contact, appeared at most as only his shadow.
Fran Fine’s father Morty was never seen on the Nanny till the end. Fran Fine’s Uncle Stanley was played by Fran Drescher’s father Morty. In the last season, her father Morty was played by Steve Lawrence, who had done a guest shot with his wife Edy Gorme in the first season.
On One Day at a Time, Schneider was always talking about his friend Moose(?), who was never seen.
I never saw this one, so I had to google to find the name of the show:
Mary Tyler Moore’s legs starred as a character on Richard Diamond, Private Detective ---- but you never saw the rest of her.
I think Kelso took her to the prom at the end of the first season. Since I happened to buy the DVDs this weekend, I’ll check into it. 
We met Duke and the rest of the gang in the episode where Niles and Frasier invest in a company that is going to bulldoze their Dad’s hangout to put up a strip mall. They end up drunk and on the floor of “Duke’s” as the wrecking ball hits. 
Back on the WCW wrestling show in the late '90s, they went with a “metafictional” approach that seemed innovative and edgy at the time, with the wrestlers rebelling against “The Powers That Be,” behind-the-scenes writers and string-pullers who had dictated all the action for a long time. “TPTB” were voiced by infamous booker Vince Russo, who was responsible for the most popular eras in WCW and its competitor WWF, but he/they were never seen. At different points, “TPTB” hired the huge, bald, tattooed Harris Brothers to serve as their enforcers, “Creative Control,” but as wrestler after wrestler burst backstage to contront The Powers That Be, we still never got a look at them/him.
Just gotta interject that when Russo took over the WCW book, the shows started going downhill fast (World Champion David Arquette, anyone?)
Wasn’t he responsible for some good stuff too, like the New Blood vs. the Millionaire’s Club, or was that still the Eric Bischoff era? Late '90s WCW is my favorite era of wrestling, but I was not yet an Internet-savvy smark, so I took everything on TV for granted, and never knew what was going on behind-the-scenes for any of it. Is there any sort of online resource that recaps that era (let’s say 1998 to the end in 2001), and explains who was in charge of writing and booking which angles?
Stan’s daughter Olivia was on the show earlier this season. Karen was jealous because Olivia and her friends liked Jack more than her. I think that the kids are supposed to be in boarding school. His son still counts as an inivisible character though.
Peggy’s mom on Married With Children; she wasn’t a long time regular, but she did stay at the house for several episodes, and Al made a lot of comments about her over the years. Never saw her, but we did get to hear her voice when she had a phone sex line as “Butter.”
Arfur Daley’s wife {'er indoors} in Minder: frequently referred to as some kind of domestic Xanthippe, but never seen.
In “3rd Rock from the Sun”, the Big Giant Head didn’t make his appearance until the 4th season when William Shatner gave an inspired performance.
I don’t think anyone has mentioned the Bubble Boy in Seinfeld. I don’t believe that we ever saw his face, although we can all imagine it from his obnoxiousness.
She also was in the Dreams episode.
You got Frank and his wife right but not Frank and Hot Lips. Hawkeye just laughed that time.