A personality thief is a person with a sever lack of knowledge in the how to of being a normal, funtioning social human being. He finds himself leaching off the personality of someone who is socially succesfully at the moment, and adopts their behavior in an attempt to fit in. However, this normally causes a backfire because of the obvious overkill present, and the pitifulness of the unoriginality.
Lashmeeeee: Wow, this is fucking clutch!
Lashmeeeee: "Failed attempt at an inside joke he's not apart of"
Victim: Lashmeeee you faggot, stop stealing my personality and get back in your fucking single!
Lugo: You fucking personality thief!