http://wordnetweb.princeton.edu/perl/we ... lter%20ego
Noun
* S: (n) alter ego (a very close and trusted friend who seems almost a part of yourself)
Alter-ego is also used to refer to the different behaviors any person may display in various situations. Related concepts include avatar, doppelgänger, impersonator, and split personality.
http://en.wikipedia.org/wiki/Alter_ego