Dr. Christina Knowlton is the main villainess from "Do No Harm," episode 1.06 of Lie to Me.
She was played by Stacy Edwards, who also appeared in The Devil's Hand as the evil Susan Brown.
Dr. Christina Knowlton is the psychiatrist for Samantha Burch, who had been missing for two months. When asked about Samantha's disappearance by Cal and Gillian, Knowlton doesn't cooperate, though she gives hints that Samantha may have been abused by her parents.
It was later revealed, however, that Knowlton was Samantha's kidnapper, and she had also abducted a girl named Heather. Two years prior to the episode's events, Knowlton's daughter, Jessica, tragically drowned. She had been abducting girls to fill the void created by Jessica's death, and regarding Samantha, the delusional villainess had been calling her, "Jessica," and brainwashing her into believing that Samantha is her own daughter. After Cal discovers that Knowlton is the kidnapper, the police search for her and Heather at Knowlton's home, finding neither of them.
Knowlton is found with Heather at the lakeside dock, which was where Jessica died. While brandishing a gun, Knowlton ranted about seeing parents "screw up their kids," and claimed that she could do a better job. After her rant, Knowlton dropped the gun and broke down before she was arrested.