the alter ego

An alter ego (Latin, "the other I") is a second self, a second personality within a person, who is often oblivious to the persona's actions. It was coined in the early nineteenth century when dissociative identity disorder was first described by psychologists. A person with an alter ego is said to lead a double life.

Alter ego is also used to refer to the different behaviors any person may display in various situations. Related concepts include avatar, doppelgänger, impersonator, and split personality.