Kathryn Merteuil is the main antagonist of Cruel Intentions.
She is portrayed by Sarah Michelle Gellar.
Kathryn was born in New York, the daughter of the wealthy Tiffany Merteuil, who raised Kathryn to be manipulative as she is, though she did not openly approve this. When Kathryn was a teenager, Tiffany married Edward Valmont whose son, Sebastian , was just as manipulative and dangerous as Kathryn. After a long-standing rivalry, Sebastian joined Kathryn in her games of manipulating the student body of Manchester Prep and anyone else. However, little did Sebastian know, Kathryn was only using him as a toy.
That all changed when a girl, Annette Hargrove, moved into town. Sebastian and Kathryn made a bet: if Sebastian can get Annette to lose her virginity to him, he could finally sleep with Kathryn; however, if Sebastian failed, he would lose his vintage 1956 Jaguar XK140 Roadster to Kathryn.
Sebastian, along the way, fell in love with Annette and lost interest in Kathryn, who then manipulated him into breaking Annette's heart. After he came to collect his "prize," Kathryn finally revealed to him that she had been stringing him along all these years and that she doesn't "fuck losers." After that, Kathryn manipulated Ronald, a mutual friend of theirs, to fight Sebastian. During the ensuing fight, Sebastian was accidentally hit by a car and died shortly after, telling Annette he loved her.
At Sebastian's funeral, Sebastian got his revenge from the grave when Annette published his journal, revealing all of Kathryn's indiscretions, including her cocaine use, to the student body and her parents. What happened after is unknown, but it can be assumed that she was kicked out of school and sent to rehab. Kathryn reputation was ruined.