Unseen characters

These characters, while significant in the story, are often minor characters. Other characters expend a great deal of effort due to their actions or existence, or talk about them a lot, or these characters accomplish off-screen actions necessary to advance the game or story. This should not be confused with faceless goons or faceless mobs, that is truly unimportant anonymous characters and/or appear only once. Also should not be confused with unseen protagonists.

Notable examples include:
The Man in the Iron Mask, the unseen character, a real person who identity may never be known.
Wilson Wilson Jr from the show Home Improvement TV show.
Maris Crane from the TV show Frasier
Little Red-Haired Girl (The Orange-Haired Girl) from Peanuts comics (Charlie Brown)
Any Adults in Peanuts comics (Charlie Brown)
Number Seven, (The 7th Cylon, the last Cylon) from the new Battlestar Galactica
Lucha Libre (Mexican Pro Wrestling), a wrestler dons a mask and will not reveal his face to the public again until defeated in a "mask match".
Beatrice Baudelaire from A Series of Unfortunate Events novel.
Band members of KISS (made extreme effort to never be publicly recorded or publically seen without their full face stage makeup)
Thompson from Citizen Kane. He's present in every scene (as the other scenes are all flashbacks), yet his face is never shown.
Undercover police must conceal their identity and will always be obscured or masked on TV when identified
Rosaline in Romeo and Juliet was originally and unseen character.
Many faiths consider is blasphemous to depict the faces of Deities or divine beings.
'knees down' characters from Looney Toons, such as Tom and Jerry's 'Mammy'. Rarely, entertainment will honor this. The Simpsons for instance, never shows the face of God. Also in Ben Hur, Jesus' face and voice are never revealed.
Numerous characters on Seinfeld TV show.
Norm's wife Vera on Cheers TV show
Bob Newhart has an extensive repartee of skits where he plays the only seen and heard character.
The Angel of Death in Final Destination.

Note that unseen characters often graduate to seen characters in later works or later revisions of the work.