When Delia is eight, Gene and Karen hire Jo Thueson as a nanny for her, and Jo tries to get through to Delia. When Delia's dog Ryder kills Jo by pushing her out a window, Karen watches in horror and faints. A traumatized Karen is taken to hospital, where Dr. Hastings secretly implants her with an embryo while she is unconscious.
Now pregnant, Karen becomes increasingly suspicious and alarmed of Delia when she realizes that Delia has never been sick at all in her life, and when Delia imitates Jo's death. She hires Earl Knight to find Delia's biological parents, and contacts Father James Mattson to discover from him what she can about Delia. Karen and Mattson discover evidence which suggest that Delia is the Antichrist.
When Earl dies after discovering Delia's identity, on the same day as Yvonne (the former nun who gave Delia to Gene and Karen), Karen talks to Gene about the incidents surrounding Delia, but Gene dismisses it as coincidence. Throughout her pregnancy, Karen becomes increasingly fearful and paranoid of Delia, and begins to believe that Delia wants Karen's unborn baby dead.
Gene hires Lisa Roselli as a new nanny for Delia, and Karen eventually gives birth to a son, Alexander. After Alexander's birth, Delia regains her sweet demeanor towards Karen, but the latter remains paranoid and suspicious of Delia.
Karen comes to believe that the Antichrist is Delia, and goes to Dr. Hastings to confront him about it. Hastings reveals that the Antichrist is not Delia, but her vanishing twin brother, Alexander. After killing Hastings, Karen returns home to kill Delia and Alexander. She kills Roselli and reaches Alexander and Delia. However, Alexander apparently uses his powers to cause Karen to shoot herself rather than kill him.
Karen was initially a loving mother to Delia, but when she began to discover Delia's dark personality and Satanic identity, Karen became paranoid and fearful of Delia.
Karen and Gene were a happily married couple, when Karen became paranoid of Delia, Gene and Karen argued over it, with Gene believing it to be all coincidence.
Karen and Hastings initially got on well, but when Karen discovered that he was a disciple of Satan, she tortured Hastings into telling her the truth about Delia, and kill him afterwards.
Karen was initially protective of Alexander. But after she discovered that Alexander was the Antichrist, and that he was actually Delia's twin, she declared him a freak and hatefully swore to kill him. However, Karen was still apparently unable to kill Alexander (though it is hinted that this was due to Alexander's influence).