Cynthia is a friend of Elaine's, who appears in "The Fix-Up".
Cynthia is set up with George Costanza. Jerry and Elaine vow that they will tell each other everything that happens over the course of the fix-up, including Cynthia's testimonies to Elaine. Cynthia and George had sex in his kitchen (claiming it is the most sociable room in the house), and breaks up with him sometime after the episode ends.
Cynthia also may have an eating disorder (bulimia), attributed to the fact that while she is eating out with Elaine, she very nonchalantly says "I'm going to go throw up", and does this in order to have room for cake. In addition, when describing her to George, Jerry mentions that Cynthia "likes to throw up," further hinting that she might have bulimia.