Kathy O'Brian is played by Emily Happe. She only appears once in the series as the second Grad student in The Cooper-Nowitzki Theorem to become infatuated with Sheldon. In the same theme as the first meeting, with Ramona Nowitzki, Kathy also was impressed with Sheldon's work, and wanted to talk with him privately and ending up agreeing to bring him take-out food to his apartment. She also already had his address. Sheldon of course missed all the implications of the encounter and when asked if he understood what happened, he said, "Yes, I'm getting a free pizza." He didn't know that he was just picked up by Kathy.
- Kathy O'Brian, like Ramona Nowitzki was also a redhead.