Sidekick Syndrome

Being unable to be the main character in your life, always looking for someone to fill the void.
It defines a person so insecure that they actually look for leaving in the shadow of their best friend that they have chosen.
Eventually they will adopt the personality of their best friend to be liked in social settings.

Gretchen in Mean Girl that would do anything to look like and be liked by Regina. And at the moment she is no longer popular, she switched to Cady. She has the sidekick syndrome, incapable of being her own person.

Sidekick Syndrome

The phenomenon by which you were never hot enough in high school to be anything more than your cool slacker best friend's wingman - until you enrolled at a top college full of other virginal nerds. Now all of a sudden you're a stud by default, or maybe you just have an average amount of pull, but it's completely new and now you have no idea how to act.

Person 1: If Fred from Freshman 15 has been dating Suzy from Sig O for two months, how come he's still so weird about kissing her in public?
Person 2: He has a classic case of sidekick syndrome. Go 'Cats.