An unseen character is a fictional character referred to but never directly observed by the audience. They are characters that are "heard of, but never heard from".
Unseen characters are a common device in drama. They are continuing characters—characters who frequently interact with the other characters and who influence current story events. Films, television shows, and stage plays make use of characters who are not seen or heard, but who have an effect on the events portrayed.
Books can feature characters who are referenced by others, but whose actions and dialogue are never directly described. The work of Voltaire, for example, included the "unseen character".
In Clare Boothe Luce's play The Women (1936), and the 1939 film based on the play, male characters (husbands, lovers, etc) are referred to but do not appear, even in photographs, and the entire cast (from stars to extras) is all female.
On Seinfeld, Bob Sacamano, Lomez and "Cousin Jeffrey" are often mentioned but never seen. The first two are friends of Cosmo Kramer, and the last is the cousin of Jerry Seinfeld. Jeffrey works for the New York City Parks Department, as Jerry is told ad nauseam by his Uncle Leo.