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 created by Peter J. Favaro, Ph.D. (released by Activision in 1986), 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. The game based on actual psychological knowledge and experience, and the authors have done their best to create a game which contains scientifical material but playable and enjoyable at the same time. It was a novel concept, to some degree similar to The Sims (but while Alter Ego let the player experience his own personality, or the consequences of changing it, The Sims lets the player to observe and explore others - hidden - peronality and their interaction in situations).
