Over at Guardian Books, they've got a great discussion going about literature's unseen characters. These are "literary creations who never make an appearance on the main stage, but whose presence nevertheless hovers over the text, influencing thoughts and actions."
Some examples that the article gives are Sauron from The Lord of the Rings, the lieutenant in The French Lieutenant's Woman, Mr. March in Little Women, Mrs. Churchill in Emma, and the eponymous Godot. There are also some fantastic ideas in the comments section.
How about you? Do you know of any literary characters (not dead) who play a significant role in the story and yet are never seen or heard?