Joyce Kim was one of Leonard's former girlfriends and a North Korean spy. The two dated for 27 days, after which Joyce defected back to North Korea, as Howard and Raj recalled in "The White Asparagus Triangulation." She has been mentioned several times and seen once. She was portrayed by Ally Maki.
In 2003, Leonard was working on rocket fuel at the time under a DARPA contract and Joyce Kim was a spy attempting to obtain information on his work by seducing him and report it back to her government. Luckily, Sheldon unknowingly stopped her, after he intruded on their fun, claiming Leonard had violated the roommate agreement. Leonard agreed to give Sheldon a 12 hour notice before having guests over, but he hadn't known Joyce Kim for 12 hours before he had her over. It is unknown when she will appear again, if ever.
Later in "The Apology Insufficiency," she is mentioned by Sheldon during Howard's job interview.