(Jungian) The dark side of our personalities that we hide from others. Typically the Hero has a strong shadow in the Villain. Some villains are literally masked (Jason in Friday the 13th, Hannibal in Silence of the Lambs).
The Shadow can also be internal: The Shadow is the repressed alter ago, a hidden presence behind the mask of the persona. In extreme cases the Shadow becomes part of a psychotic split within the character (Norman Bates in Psycho, Serial Killers).
