(1) A trusted friend or personal representative. (2) The opposite side of a personality.
The White House chief of staff is a political alter ego; he knows, or should know, who and what the President considers important.
In Latin, alter ego literally means "second I." An alter ego can be thought of as a person's clone or second self. A professional alter ego might be as trusted aide who knows exactly what the boss wants done. A personal alter ego might be a close friend who is almost like a twin. Alter ego can also refer to the second, hidden side of one's own self. In Robert Louis Stevenson's classic The Strange Case of Doctor Jekyll and Mr. Hyde, Dr. Jekyll is a good-hearted, honorable man. But after taking a potion, his alter ego, the loathsome and diabolical Mr. Hyde, takes control over his personality.