Unseen character
An unseen character is a fictional character referred to but not directly observed by the audience.
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.
Examples
Novel
In Miguel de Cervantes's Don Quixote (1605 and 1615), the protagonist's lady, Dulcinea del Toboso, is often referred to but never appears.
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, the French author, Voltaire, included unseen characters in a few of his plays, including Le Duc d’Alençon, and L’Orphelin de la Chine.
Rosaline in William Shakespeare's Romeo and Juliet is never seen, but is only described.