Karen Smith is a main character in Mean Girls. She is portrayed by Amanda Seyfried.
Karen is one of the most popular girls at North Shore High School. She is the third member of the clique known as "The Plastics" , a group of girls who dominate with their beauty and general aura of intimidation. Karen is the most beautiful in the group even more beautiful than Regina George, Gretchen Wieners, and Cady Heron. She also is very promiscuous and will make out with any boy she crosses paths with.
Karen seems to be the kindest of the Plastics; she's rarely seen insulting people or belittling others as Grecthen and Regina often do, and she seems to play the part of the "quiet little mouse" who doesn't outwardly contribute much to Regina's dominating meanness in public. Yet, despite her kindness, Karen is clearly the slowest of the three, making her an easy target for Regina to trivialize and manipulate. Despite Regina's actions she still loves her friends and wouldn't have it any other way.
Besides being the ditz of the Plastics, Karen also appears to have a childlike innocence about her. Her playful obliviousness to what's going on around her often causes her to make light of serious matters, such as her disciplinary trip to the principal's office after the discovery of the Burn Book. While the principal is trying to discuss the issue with her, Gretchen, and Cady, Karen abruptly begins to laugh after he reads an offensive quote from the Burn Book. Her smile falters instantly when the principal scolds, "Miss Smith, this is no time to be laughing".
Karen's innocence even pervades into her sex life; during the Halloween party, she makes it known to Gretchen that she's kissed her own cousin, Seth, and thoroughly enjoys it. When Gretchen points out the inappropriateness of this, Karen tries to reassure her that he's only "her first cousin", and begins to ramble on in an incorrect explanation of family trees, unable to believe that her actions are anything but innocent. Gretchen eventually interrupts her, and Karen finally concedes that her intimate activities with her cousin are "so not right", though she's seen running toward him (for an implied make-out session) later on during the party.
Ironically enough, Karen is fully aware of her dim and naive outward persona. After Regina yells at Karen for her tactless suggestion of taking a trip to Taco Bell, Cady tries to comfort Karen, only to have Karen admit to her own "stupidity". In a defeated tone, Karen confides in Cady that she's "failing almost everything" in school, and believes that her only noteworthy talents are her abilities to put her whole fist in her mouth and to predict the weather via her breasts. Cady humors Karen to spare her feelings, but makes it quite clear to the audience through her facial expression that she does not take Karen seriously at all.
Toward the latter half of the movie, the "friendships" among the Plastics become strained, and Karen, despite her generally kind nature, assists Gretchen in ganging-up on Regina one day at lunch. During this scene, Cady and Regina are already sitting at the lunchtable when Gretchen and Karen arrive and immediately confront Regina on her choice of clothing. The three girls have a quick banter over Regina's illegal sweatpants (which are not to be worn on Mondays), during which Karen swiftly asserts, "So that's against the rules, and you can't sit with us!" It is unlikely that Karen would ever stand up to Regina without the help of Gretchen or at least Cady.
After the Burn Book is discovered by all the other junior girls at school, Karen, Gretchen, Regina and Cady must join their fellow female students in a peacemaking workshop run by Ms. Norberry in the gym. When the girls are all required to make public confessions and apologies onstage, Karen displays her lack of tact once again by announcing one of Gretchen's most embarrassing moments, and then apologizes in retrospect for embarrassing her.
When the Plastics finally break up at the end, Karen becomes a weatherwoman (using her "special talents") and is also seen braiding another girl's hair as Regina walks by with her own new friend. It's implied that Karen and Regina are still on good terms with one another when Karen looks up from her hair-braiding, waves and says congenially, "Regina!"; Regina acknowledges Karen's greeting with a friendly wave and smile.