Karen was a woman whom George dated. He was fairly serious about her and Jerry helped him realize that he loved her (because he would clean the bathtub before she came over). Karen was portrayed by Lisa Edelstein and appeared in two episodes of Season Five.
When George realized he loved Karen, Jerry asked if there was any minor flaws with the relationship that he needed to iron out. George confessed that he couldn't orally please Karen, and then extrapolated his own confession to wonder if she faked all of her orgasms. He couldn't perform sexually with her on their next date. However, after eating some mango from Kramer, he felt his equipment move so he rushed to see Karen. After finishing, he told her that it was alright that she faked that time. He then made fun of her orgasm, assuming it was fake. She then furiously kicked him out of her apartment. ("The Mango")
Karen also appeared in "The Masseuse". In order for Karen to get a window into his date personality, George arranges for Karen and George and Jodi and Jerry to go to dinner. At dinner, George noticed a look from Jodi, and later George confided in Karen that he believed Jodi didn't like him because of Jodi's behavior. Though Karen tried repeatedly to assuage George of his fears and minimize his neuroses, even going as far as to remind George that not everyone in the world had to like him, inevitably, George became increasingly obsessed over Jodi's dislike toward him. Karen gives George an ultimatum: it's either Jodi or Karen. George ended up leaving Karen's apartment, saying, "I know I care for you, but I can't stand it when somebody doesn't like me." Then Karen replied, "Now I hate you!" George just shrugged and said, "THAT I'm used to."