Alter ego
See also Alter Ego (game) for the Commodore 64 gameAlter 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).