You like to make fun of us, but we are more powerful than you think.
~ Kevin Wendell Crumb, as Dennis, talking about his other personalities.
Villains who have a split personality, or seem to have more than one personality. This can result from mental illness (such as Norman Bates), an experiment gone wrong (such as Marvel Comics' Lizard), a curse (like that upon Gollum), multiple beings condensed into one form (such as Naraku), etc. It is possible that one personality is good while another is evil in some cases (e.g. Dr. Jekyll and Mr. Hyde), but cases which the two personalities are both evil do exist as well (e.g. the original Norman Osborn and Green Goblin).

These villains are rather unique as a storytelling device when one of the personas is good since, unlike Possessed/Brainwashed villains, Villains with Dual Personalities often cannot have their second selves exorcised or be snapped out of it. Even when both/all personalities are evil, the fight for dominance and startling shifts in motivation and method keep the heroes guessing on how to deal with them.