In the unseen original pilot, Katie is the show's female lead. She is a girl that Leonard and Sheldon discover sitting on a curb, crying about the breakup of her relationship with her married boyfriend. They eventually invite her to stay with them. Katie has quite a few rough edges. She tells Leonard rather flippantly that she was sleeping with her mother's new husband. In order to explain to Leonard why she feels the need to go out and get drunk, she blithely erases a whiteboard in the apartment, much to Leonard's horror, and diagrams her explanation in "nerdish" ("Let's say that Katie is 'K' ...").
Katie was played by Amanda Walsh