Alter ego

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).

Also the name of a personality game for the Commodore 64, which allowed you to make decisions for an imaginary person (being therefore your alter ego) and showed you what possible consequences that decisions could have on that person. It is hard to tell, whether and to what degree the game was based on scientific fact, but it was a novel concept, to some degree similar to The Sims.