Television shows and stage plays sometimes include continuing characters who are never seen or heard by the audience, but only described by other characters. Radio shows and plays also feature characters who never speak, and books feature characters who are merely referred to.
Television shows: Examples
Heard but not seen
Neither seen nor heard
(may be partially seen on rare occasions)
Heard but never completely seen
Unseen characters who were spoken of for a long time and then eventually seen
Stage Plays: Examples