Karen was the best friend and former babysitter of Sandy Cooper, as well as the wife of David Waterford, a political candidate who Sandy met under the name, "Sam," and engages into a sexual relationship, but doesn't learn until afterwards that he is, in fact, Karen's husband. It is revealed that David had a previous fling with Ann Logan, who was found dead in a wooded area, and Sandy suspects that David killed her to keep her quiet. After Sandy goes to a secluded cabin to save Karen from David, she confesses to the affair to Karen, only for Karen to grab a knife and lunge after her, revealing herself as the actual killer.
Karen knew about David's affair with Ann, so she tracked them both down on several occasions. She invited Ann to take part in a photo session, only to chase her down the aforementioned wooded area, bludgeon her with a rock, and choke her to death. After finding out about Sandy's affair with David and how she was investigating Ann's murder, Karen killed a man that Sandy had meet who knew information that would expose her. She also cut the brake lines to Sandy's car in an attempt to kill her. Regarding Karen's evil motive, she planned on living the good life after her husband was elected, and she had been eliminating anyone who got in the way of that. Karen chased Sandy outside the cabin with the knife, only to be shot and killed by Sandy after David stopped her.
- Karen Waterford is similar to Alice Walters from Secret Liaison: both are evil wives married to budding politicians, and are willing to kill to make sure their husbands are elected.
- "You know how behind every great man is a woman? I'm going to be that woman." (Karen's line before going after Sandy)