Minor Characters
Throughout the years of the show, more characters have come into the show more often. There names are, Leslie Winkle, Bernadette (Shown on top left of the picture), Amy Farrah Fowler (Shown on top right of the picture), and Stuart Bloom. Leslie Winkle is an experimental physicist who works in the same lab as Leonard, and is arch-enemies with Sheldon. Bernadette works with Penny at the Cheesecake factory. Her and Howard meet and have connection. They begin to date, and sometimes they break up, but eventually they get married. Amy Farrah Fowler is a neuroscientist, who meets Sheldon through an online dating website after Howard and Raj made an account for him. Eventually her and Sheldon become friends because of their similar interests. After time they decide on an "agreement" of being boyfriend/girlfriend, which specifies the things they can and cannot do. Stuart Bloom is the owner of the comic book store that the guys occasionally go to. All these characters occur often in the show, but were originally not the main characters.
Sources: Google.com/images
http://en.wikipedia.org/wiki/The_Big_Bang_Theory
http://en.wikipedia.org/wiki/The_Big_Bang_Theory