In L.A. Story, a character named Harry Zell is mentioned a few times but never seen. He was originally supposed to appear in the movie (played by John Lithgow), but all his scenes were cut.
The Thane of Cawdor does not appear in Macbeth, but his execution off-stage and Macbeth’s inheritance of his title is important to the early plot, as it confirms the prophesy of the Three Witches and makes Macbeth think the kingship is also within his grasp.
If I remember correctly, either in the “bug out” episode, or the final episode, the ghost was making announcements even after everyone had left.
For a supposed comedy, I’m surprised te producers never made fun of this. Did it just not occur to them that there was no one that could be making the announcements? Probably not, if they were added in post-produciton, no one ever noticed.
I think I read somewhere that the name was borrowed from a Tour de France competitor who was always the Tail-end Charlie.
That was my first thought too - and in the same authors’ Hi-de-Hi! there was Miss Cathcart the cleaning supervisor.
And let’s not forget Sheridan Bucket and his friend Tarquin.
Come to think of it, it was a trope in the ancient BBC radio show Take It From Here, and its sketches with the Glum family: we never got hear Mrs Glum.
There’s a Disney fan theory that it was Gaston.
I use antlers in all of my decorating 
Two movies, Oscar and Garbo Speaks ****almost qualify. Oscar stars Sylvester Stallone whose daughter tells him that she is pregnant with Oscar’s baby (she’s lying), who appears briefly at the end. In Garbo Speaks , Greta Garbo appears near the end, but only from behind, and she never speaks on camera.
There was another episode where she got a job in the restaurant upstairs, and her voice is heard calling down to Norm from above.
It’s up to the OP, but personally I think that voices from offscreen ought to count as an appearance.
The voice on the tannoy at MASH appeared
Charlie speaking over the intercom appeared
Carlton the doorman speaking over the entry phone appeared
In my opinion.
And by the way, there was a Carlton Your Doorman animated pilot which is a definite appearance.
Now that I think about it, there was an episode where a soldier’s ghost wanders around camp.
If were adding never heard radio characters, there’s the lovely scorekeeper Samantha from Sorry, I Haven’t a Clue.
If that counts, does The Banker from Deal Or No Deal count?
A long tradition on classic TV shows:
Norm had “Vera”, Niles Crane had “Maris”, and Buddy Sorrel had “Pickles”, though I think we got to see her a couple of times early in the Dick Van Dyke series.
Oh, and “This is Carlton your doorman…” (Ahh, I see Cal mentioned him)
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Agreed! If anyone hasn’t seen it… do so!
Don’t forget Sven…
Darth Plagueis The Wise does NOT appear in The Star Wars Prequel Trilogy despite being responsible for the birth of Anakin Skywalke (Darth Vader) and being mentioned his apprentice Senator Palpatine (Darth Sidious).
For that matter Jedi master Sifo-Dyas was also never SEEN and he was responsible for the creation of the clone army
He does show up in the final scene - he just missed the performance. Or was that just a shadowy figure in a car?
To those saying Harvey doesn’t appear in the film, I counter with this bit of evidence.
Now, I happen to think letting the audience see this painting is one of the movie’s only missteps, since it makes concrete something that should only exist in each viewer’s imagination. And I don’t know about you, but that’s definitely not how I imagine Harvey. But I think it clearly disqualifies the film Harvey from the OP’s specifications.
My contribution to the thread (and I’m surprised that I’m the first to mention him) is Floyd Thursby in The Maltese Falcon, both Dashiell Hammet’s book and John Huston’s 1941 film with Humphrey Bogart.
Thursby was Brigid O’Shaughnessy’s partner before the story starts, and she sets Spade and Archer after him, claiming he has seduced her (non-existent) sister. Archer starts tailing him and is killed by Thursby. Shortly after that, Thursby is killed, off-camera, by Wilmer Cook.
The OP’s go-to example allowed “a life sized painting of her.”
It certainly does not.
As I point out in the OP, a painting of the never-seen Marguerite appears in the 1972 film version of Sleuth, yet that film is part of my list. “appearing in a painting” isn’t the same as “appearing”
Here she is:
Rosemary’s Baby. The titular youngster looms large but is never seen (which increases the horror—Farrow’s reaction is brilliant).
Frank Spencer’s much-mentioned mother never appeared in Some Mothers Do ’Ave ’Em.