There are a number of TV characters who are only known by what the other TV characters in the show say about them. These are the UNSEEN CHARACTERS OF TV. Within a short time, I came up with a list of unseen TV characters.
In no particular order:
Heather Sinclair, frequently trash talked girl at Degrassi TNG.
Lars Lindstrom, the dull, Swedish husband of Phyllis. This unseen character was killed off (off screen, of course) in the pilot of the PHYLLIS sitcom.
Sara & Juanita, the phone operator and diner waitress, respectively, in Mayberry. Juanita was the "go to girlfriend" if Barney's regular girl was not available. In one of the episodes, Barney recited a poem he had written to her (she is unseen, as he talks to her on the phone):
Lovely, dear Juanita,
From your head down to your feet,
There's nothing half so sweet,
As Juanita, Juanita, Jua-neet.
Oh, there are things of wonder,
Of which men like to sing.
There are pretty sunsets and birds upon the wing,
But of the joys of nature,
None truly can compare,
With Juanita, Juanita, she of beauty beyond compare.
Juanita, Juanita, lovely dear Jua-neet.
Reverend Feltcher, who Archie would mistakenly call "Fletcher."
Edith (correcting him): Fletcher.
Charlie from CHARLIE'S ANGELS was not seen.
Maris, maybe the best unseen character.
Carlton Your Doorman
Belker's Mom from HILL STREET BLUES, who would always call when he was booking a suspect.
Orson (Mork & Mindy)
Now we get into a murkier sub-area. There are certain characters that referenced for years and sometimes in some shows, they eventually are cast with a real actor and come to life. For instance:
Johnny Paul Jason, a frequently referenced friend of Opie. The character finally appeared in a handful of episodes beginning 4 years into the series.
Mrs. Columbo was the pretty much always referenced wife of Lt. Columbo. While she never appeared on any of the Peter Falk-starring programs, there was a shortlived series titled MRS. COLUMBO (sans Falk).
Pickles was the memorable name of Buddy Sorrell's wife in THE DICK VAN DYKE SHOW. She was eventually seen as the series went on.
And, of course, the Big Giant Head -- the frequently named leader in THIRD ROCK FROM THE SUN, who finally showed up, played by the and only William Shatner. My favorite clip: Shatner & Lithgow first meet and reference the same TWILIGHT ZONE episode.
After playing this little game, I found this:
Wikipedia's Unseen Characters page.
Oh well! It was more fun to come up with these on my own. If you have any to add, or if I made mistake, please comment.