In the original unaired pilot of The Big Bang Theory, Katie was the female lead character later replaced by Penny.
Katie was played by Amanda Walsh and was conceived as a tougher, more street-hardened counterpart to the socially awkward scientists Leonard and Sheldon. The pilot’s basic setup involved the two men meeting Katie and inviting her into their apartment, but her personality and the episode’s darker tone tested poorly with audiences.
The creators reworked the concept for a second pilot. Katie disappeared, Penny was introduced as the warmer, more approachable neighbor, and the ensemble gained the characters who became familiar regulars, including Howard and Raj. Kaley Cuoco ultimately played Penny in the broadcast version.
A common mix-up is confusing Katie with Gilda, another woman from the unaired pilot. Gilda was the scientists’ colleague and friend, not the central female character replaced by Penny. The original pilot therefore functions as an alternate early version of the series rather than a normal unaired episode of the final show.