An alter ego is a second personality that lives within the same person. A person who leads a double life, with a completely different career, family, or personality, is said to have an alter ego. Most superheroes are alter egos of regular people.
The Encyclopedia Britannica says Sigmund Freud first coined the term "ego" to mean the portion of the human personality that is experienced as the “self” or “I” and is in contact with the external world through perception.
The Alter Ego
From Latin, literally meaning, "second I," the alter ego is a separate and distinct personality created by the individual--usually to live out a better, or fantasy, version of the self.
The Merriam-Webster Online Dictionary says the noun alter ego means a second self, as in a trusted friend--the opposite side of a personality, or a counterpart.
In the analysis of literature, the term alter ego can be used to describe characters that are psychologically identical, or characters who intentionally represent the behaviors, thoughts or ideas of the author.
Famous Alter Egos
Some of the most famous alter egos are Dr. Jekyll for Mr. Hyde, Superman for Clark Kent, Charlie Brown for Charles Schultz and Hannah Montana for Miley Cyrus.