Indoors

I particularly liked the voice:

"Mwuahwhabuahwuawuwablaahwahh"
"Yes ma'am"
"Mwuamuamuamuamuwahwahwamuaamuah" etc etc

Probably sums up just how much attention was given to teachers by children when they were dispensing with advice.

Plus, a friend of mine used to sound just like her when he was drunk.

Here are just a few of my favourite facts about her:

She has abnormally rigid vertebrae, slightly webbed fingers and an underbite. Her skin has no pigmentation. She is too light to activate a whoopie cushion or leave footprints in the snow. She cannot ride a horse because her quadriceps are so tight she cannot straddle anything larger than a border collie.

who must be obeyed - Rumpole

Willie Eckaslike - Corrie

Margaret in the Little Britain shop sketches, which were much funnier than the gross-out stuff that was common in later series.

"Margaret! Have we got any pirate based games..." etc.

In People Like Us, one of the funniest things ever on TV and radio IMHO. To be fair, he can be glimpsed occasionally (briefly reflected in a shop window, say) but he's definitely an off-camera character.

...Carlton, your doorman.

The next door neighbour in "Home improvements"

Only ever saw his eyes.

And one for people "of a certain age"

"Sylvia" -from the Charlie Drake 1960s sitcom "Who is Sylvia?"

Once upon a time there were three beautiful girls who went to the police academy

in Hill Street Blues

Mork calling...

...from The Big Bang Theory is giving Maris a run for her money.

I nearly died laughing when Niles bought a whippet and it was deduced psychologically that it was a Maris subsitute. When they put a pill box hat on it Niles fainted. It was true!

it's getting a bit busy down here.

Do we ever see the dreaded Vera, Norm's wife in "Cheers"?

we ever got a glimpse of Mrs Slocombe's pussy

(apart from that infamous out-take)

the dwarf (or midget?) forklift truck driver in The Office.

Dave's former fling and Denise's nemesis, Beverley Macker

For your consideration...

The Maid from the Fred Quimby T&J cartoons - only ever see her feet.

But Sabrina Thwaite sounds like she's a bit of a gal.

in The Prisoner.

Oft referred to - never seen. Very unnerving.

Since the maid from Tom and Jerry is taken, may I submit for your consideration Reggie Perrin's mother in law, who (in the original series) was only seen through Reggie's imagination, and always as a hippo accompanied by tuba music.

legendary lawyer on Neighbours in the mid 90s. Anyone who had any trouble would always go and see Philippa, but we, the viewers, never did. Mrs Mangle (a soap name worthy of Dickens) had a husband called Len, who we never saw, only for him to turn up years later, thus ruining the illusion.

...Jim Henson and Frank Oz, unseen characters of a sort...

on Seinfeld.

Was always doing well working for the NY Parks Department, according to Uncle Leo. Had a face like a horse.

Bob Sacamano from Seinfeld.

Followed by Captain Mainwairing's other half.

who owns the laundrette in Albert Square.
The Chief in 'Stop The Pigeon', only ever heard berating Dastardly on the phone.

I once spent an afternoon with Reg Varney at his house in Devon, and several times during the visit he went upstairs, he said, to see his wife. I heard and saw neither hide nor indeed hair.

But I'll go for Marni Nixon, particularly in West Side Story and My Fair Lady. (She had a bit part as a nun in Sound of Music, but Julie Andrews didn't need any help!)

..... there's landlady Marsha's daughter Amber - usually a blur of rushing fetlocks, muffled arguments and slammed doors. Her opinions on The Phantom Menace remain unknown.

Edit : I might also add Mr Doyle from Father Ted - his one mention by his tea-dispensing missus had the triumvirate of clergy leaning forward in increased interest at this Revelation that wasn't included at the end of the big book, to which she comments : "I've said too much!"

BR
FT

....Snuffleuppagus *sort of* counts - nobody on the show could see him except for Big Bird (at least for a while - I *think* he eventually became see-able to everyone else: perhaps someone could enlighten me!)

Anyway, it's a good excuse to see him singing a madrigal with the lovely Judy Collins.

By the way, Mens- you can't possibly tell us you spent an afternoon with Reg Varney and leave it at that! we need details!

who Bill and Ben had to hide from in The Flowerpot men.

who really should learn to post himself,as should Ella en casa.
Peggy Bundy's mother (Married with Children)butt of many of Al's best jokes.
"Monkey" Harris (Only Fools and Horses) Seems to be in league with Del but is never seen.
Mrs. Swainey (One foot in the Grave) who Victor thought didn't exist.
Joe Maplin (Hi-De Hi!) always mentioned,never seen.
from Mrs Crout
Diane (Twin Peaks)who Dale Cooper constantly refers to.

It's not TV and he's not unseen but when it comes to unforgettable characters not physically present you may bow down to the guvnor:

She was almost seen, she gets into the bunk above Mainwaring in the air raid shelter and the mattress of course then almost touches Mainwaring on the nose.

Corkys wife in Sykes was never seen but she was heard. She spoke in a totally unintelligable way.

has a few:

Morrie Hardaker is mentioned quite often in the original series but never seen;
likewise the boss of the electrical firm, Old Darby;
Terry apparently has another sister, although we know little about her;
Terry's ex-wife and the infamous Deirdre Birchwood (who both, admittedly, belong in the past - not to mention, in the case of the former, in Monchengladbach) are also frequently referred to but never seen.

Diane.

in Marion And Geoff. Not to mention Keith's "little smashers".

The early evening Saturday slot had a series called Garry Halliday which featured an unseen arch-villain called The Voice.
Also - The Lovely Samantha from I'm Sorry I Haven't A Clue (the mind boggles!).

The Fucking Stig.

Twat.

One of the many many reasons I stopped listening to this was the unheard characters, a pathetic in-joke with the listeners. People always saying "Oh, you can't get a word in once that Derek Fletcher gets going". Har-de-bleeding-har.

Or were they just the cheapest, yet curiously most effective, effect ever?

Kids programmes are full of unseen characters: i.e. the kids' parents. The first example that springs to mind is Charlie and Lola's mum and dad. I'm sure that there are many others.

Not that they are in any way as memorable as the unseen spouses and other interesting folk mentioned above.

The mother from the family in Absolutely. You could see her back but never her face.

We never see Navid's amusing wife.

in the recesses of my memory since this thread started, and has only just come to the fore. There was an episode of "M*A*S*H" in which Hawkeye invents a fictional soldier called Tuttle. By the end of the episode, when they're holding a memorial for him, everybody claims to have met him.

EDIT: Turns out there's a Wikipedia page on the episode. http://en.wikipedia.org/wiki/Tuttle_%28M*A*S*H%29
It also turns out that this is the only episode in which the character Sparky - usually on the other end of Radar's radio - appears.