The Alter ego reference article from the English Wikipedia on 24-Apr-2004
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Alter ego

See also Alter Ego (game) for the Commodore 64 game
Alter ego: (from latin, 'other self') Another self. A different personality or persona (esp. if similar or opposite). Can also be used about people who are identical psychologically, common in literature analysis and comparison.

Mr. Hyde was Dr. Jekyll's evil alter ego (same person, different personalities). Asterix is Mickey Mouse's alter ego (different people, same personality).