Leslie is the female version of Leonard and the two even dress similarly.
In season one, Leonard feels that Penny is way out of his league so he asks Leslie out instead.
He works as a theoretical physicist at Caltech and he is best friends with Leonard who is also his roommate.
He and Amy have an awkward relationship and have been dating since season 5.
Most of the actors on the show already had acting experience before they made it onto The Big Bang Theory, however, for most of the stars, the show launched their careers into an upward spiral of success.