Now this is interesting - I also got a nickname in highschool from a popular kid that helped me gain an identity in other people's minds and be much more social and involved, but I don't think it was an alter-ego. I think it helped make me memorable to other people while giving me a confidence boost (as the nickname was complimentary), but I think that it just brought out the real me. I don't go by that nickname since I left highschool, but I feel the same as I did then. It was more about getting over shyness and being included than being someone else. I suppose it varies from person to person!
Edit: The super pervasive nickname is a fun phenomenon, eventually even the teachers and principal called me exclusively by the nickname.