The "unseen character" is a device that has been used in many television programs. It's usually reserved for the wife or mother of a peripheral character, but has been used a number of times in many ways throughout television history. In some cases, the character is mentioned so often that they become a living and breathing part of the show, even though you never see their face. The very idea of the character becomes real, even in the cases where they aren't given a physical appearance.
The idea of the unseen character is nothing new. The character of Rosaline is never seen in Romeo and Juliet, but without her role, Romeo would have never first laid eyes on Juliet.
Take a look at these faceless television characters and try to imagine their respective programs without their presence. In some cases they may not be as important to the plot as Rosaline, but the show would lose something without them.
10 Ugly Naked Guy - Friends
On Friends, the characters Monica and Rachel (and later it would be Monica and Chandler) had a perfect view in an apartment just below and across the street from their own. This allowed them a glimpse into the not so private world of "Ugly Naked Guy", a man who walked around his home in the nude with his curtains open. The character was a running gag through the bulk of the show's run from 1994-2004.
The closest glimpse we were ever given of the character was when Ross was trying to sublet Ugly Naked Guy's apartment by eating muffins with him in the nude. Viewers were only able to see the character from the back.
9 Diane - Twin Peaks
Through the two seasons we were given of Twin Peaks, a running bit was special agent Dale Cooper recording messages for the unseen character of Diane. Generally these messages had to do with the case of a murdered girl in the town of Twin Peaks, but almost just as often they'd be Dale Cooper's general musings with messages like, "Diane, I'm holding in my hand a small box of chocolate bunnies."
There were many theories as to who Diane was through the course of the show, but in the prequel movie it seemed pretty clear that she was Dale's assistant at the FBI, as he says hello to the character at work one morning. We still never actually see the character.
8 Dr. Claw - Inspector Gadget
During the animated series run of Inspector Gadget, the lead villain known as Dr. Claw, was a nod to Ernst Stavro Blofeld, a James Bond villain that originally was never seen, except for his arms stroking a cat. It's this same image we were ever given of Dr. Claw. In fact, we didn't even know the first name of Dr. Claw until it was humorously revealed that his first name was George and that Claw was his legitimate last name.
Sadly, when the live-action film came out in 1999, they made a point for him to actually be seen (which eventually happened to Blofeld as well), and even gave the character a robotic claw arm. They also renamed the character Sanford Scolex for reasons unknown.
7 Heather Sinclair - Degrassi
She was the most popular girl at Degrassi High. The "queen bee" type if you will. She was the object of affection to many young men at Degrassi High, she was a straight-A student, popular, and a few words from her mouth could ruin the reputation of a student at Degrassi. She was even the arch-nemesis of a few of the main characters. All of this, and yet the character was never seen.
In later seasons, her sister Holly J. Sinclair, as well as her mother, would be seen on the show, but Heather Sinclair would remain one of the only real mysteries in the High School drama.
6 Grown-ups - Charlie Brown Specials
On the Charlie Brown television specials, the adults were typically absent. When grown-ups like parents or teachers were necessary to move a story or gag along, they were always off screen and spoke in a series of unintelligible gibberish. Some thought this was commenting on the fact that the children weren't really listening, but it's actually a bit of a gag that plays off the original comic strip.
To fit the children in the panels of the strip, the "camera" was put at the level of the children, and anyone taller was just left "off camera" and the dialogue of the Peanuts Gang provided exposition. In the show, they would use the "wah wah" of the adult dialogue just so viewers would understand that someone was talking. The characters still provided exposition for the dialogue of adults.
5 Nanny - The Muppet Babies
The Muppet Babies was a very popular children's cartoon that ran between 1984 and 1991. During this time the character of Nanny was in every episode, but her face was never seen. The reason for this was to portray the world from a children's "looking up" point of view. Household furniture was drawn to feel enormous and the faces of adult (typically human) characters were never seen as to keep the Muppet Babies in frame. This is similar to the portrayal of adults in the Peanuts specials.
The only exceptions to this rule were for cameos from "human" Muppet characters like Statler and Waldorf.
Fun fact: The voice of Nanny was appropriately provided by Barbara Billingsley, who played the quintessential "television mom" June Cleaver on TV's Leave it to Beaver.
4 Danny - The X-files
Whenever Fox Mulder needed information to help solve a case, he would always get a hold of Danny. There were rumors that Danny was another Daniel character from the show, but Carter laid that theory to rest. In later episodes, Scully and even Doggett would use Danny as an informant.
There was actually a flashback episode planned that was supposed to show Danny in the flesh, but due to cutting the show for time, the scene was taken out.
Even executive producer R.W. Goodwin is in the dark on Danny. In an interview, Goodwin said that he thought Danny was supposed to be Fox Mulder's brother. This is another theory that was put to rest when show creator Chris Carter confirmed that the Danny character's last name was Valladeo.
3 The President of the United States - Veep
We're not sure if this is intentionally done for a gag or not. It could just as easily be an attempt to keep the show "timeless" by not casting the President. Regardless, it is pretty humorous that the Vice President of the United States is seemingly never able to actually get face time with the President. It's also humorous that during the first couple seasons of the show, the Vice President doesn't seem to have very much important work to do.
Vice-presidential Historian Joel Goldstein was asked to comment on the reality of the situation presented in Veep and said that a VP would have face time with the President during every important issue. Still, it's a good comical element for a television program.
2 Mrs. Wolowitz - The Big Bang Theory
Mrs. Wolowitz has never been seen but has definitely been heard in the episodes of The Big Bang Theory that she "appears". The character is known for her very loud and obnoxious voice.
As the show has gone on, we have learned a little more about Mrs. Wolowitz. We now know her name is Debbie, and we have even seen her whole body. The catch is, we were given this view of Mrs. Wolowitz in an overhead shot of her son Howard's wedding. Her face remains to be seen.
1 Bob Sacamano, Lomez, Cousin Jeffrey, and George Steinbrenner - Seinfeld
It's quite possible that Seinfeld holds a record for the most uses of the "unseen character" trope.
Sacamano and Lomez were names we heard dropped by the Kramer character on a number of occasions. Kramer wasn't exactly on the level and he seemed to get around, so it makes sense that he had another group of shadier characters he hung out with.
Cousin Jeffrey was always name dropped by Jerry's Uncle Leo, who also made a point to mention that Jeffrey worked for the New York City Parks Department every chance he was given.
Steinbrenner was an owner of the New York Yankees and George Costanza's boss on the show. He is more of an honorable mention for this list. While his face is never seen on the show and his voice was provided by Larry David, Steinbrenner is a real person, so it isn't hard to find out what he looks like.