Unseen character

An unseen character in theatre, comics, film, or television, or silent character in radio or literature, is a character that is mentioned but not directly known to the audience, but who advances the action of the plot in a significant way, and whose absence enhances their effect on the plot.
History.
Unseen characters have been used since the beginning of theatre with the ancient Greek tragedians, such as Laius in Sophocles' "Oedipus Rex" and Jason's bride in Euripides' "Medea", and continued into Elizabethan theatre with examples such as Rosaline in Shakespeare's "Romeo and Juliet". However, it was the early twentieth-century European playwrights Strindberg, Ibsen, and Chekhov who fully developed the dramatic potential of the unseen character. Eugene O'Neill was influenced by his European contemporaries and established the absent character as an aspect of character, narrative, and stagecraft in American theatre.
Purpose and characteristics.
Unseen characters are causal figures included in dramatic works to motivate the onstage characters to a certain course of action and advance the plot, but their presence is unnecessary. Indeed, their absence makes them appear more powerful because they are only known by inference. The use of an unseen character "take[s] advantage of one of the simplest but most powerful theatrical devices: the manner in which verbal references can make an offstage character extraordinarily real [...] to an audience," exploiting the audience's tendency to create visual images of imaginary characters in their mind.
In a study of 18th-century French comedy, F. C. Green suggests that an "invisible character" can be defined as one who, though not seen, "influences the action of the play". This definition, according to Green, would rule out a character like Laurent (Lawrence), "Tartuffe's" unseen valet, whose sole function is merely to give the playwright an opportunity to introduce Tartuffe.
Unseen characters can develop organically even when their creators initially did not expect to keep them as unseen, especially in episodic works like television series. For instance, the producers of "Frasier" initially did not want to make the character Niles Crane's wife Maris an unseen character because they did not want to draw parallels to Vera, Norm Peterson's wife on "Cheers", of which "Frasier" was a spin off. They originally intended that Maris would appear after several episodes, but were enjoying writing excuses for her absence so eventually it was decided she would remain unseen, and after the increasingly eccentric characteristics ascribed to her, no real actress could portray her.
Examples.
Theatre.
Unseen characters occur elsewhere in drama, including the plays of Eugene O'Neill, Tennessee Williams, and Edward Albee. Author Marie A. Wellington notes that in the 18th-century, Voltaire included unseen characters in a few of his plays, including "Le Duc d’Alençon" and "L’Orphelin de la Chine".