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Since I will no longer be playing games here, I will go ahead and reveal the answer here and declare open floor: Each of these shows featured a character who was often referred to and talked about but never seen onscreen.
On All My Children, Phoebe Tyler had a bridge playing partner named Juanita Ramsey who never appeared onscreen.
On The Andy Griffith Show, whenever Barney Fife found a moment away from Thelma Lou, he was making long distance phone calls to a waitress in the neighboring town of Mount Pilot, whose names was also Juanita.
On Cheers, Norm always talked about his wife, Vera but we never actually met her. She did appear on one episode, but she got hit in the face with a pie so we still never saw her face. She was allegedly played by George Wendt's real life wife.
On Frasier, Niles (David Hyde Pierce) was under the complete control of his domineering wife, Maris, but we never saw her.
And, of course, on Will & Grace, Karen Walker (Megan Mullally) was married to a two ton millionaire who we never saw named Stanley Walker.
OPEN FLOOR