The villainous Amanda
Amanda (Shanna Collins) was a villainess from "Blood Relation," episode 3.04 of Medium (airdate November 29, 2006). She was introduced as a friend of Jack Walker, who was revealed as the lookalike great-grandson of notorious serial killer Dr. Charles Walker.
Charles' victims during his spree were young girls, and after a pair of previous encounters in the series, Allison DuBois began having dreams about Charles' past (including the death of his wife, Maura, while she was giving birth to their son), which led her to believe that Amanda was her next target upon seeing her. Amanda was approached by Allison, who later saw Jack and (believing that he was Charles and that Amanda could see him) panicked and called police.
After Jack was taken into custody, it was revealed that he and Amanda met when the latter collided with Jack in a car accident, and Amanda had been paying for the repairs in installments. They had spent two months together, all the while, Jack had revealed to Allison that he had been hearing Charles in his head, as he had been voicing his murderous fantasies to Jack, who turned to alcohol to eliminate the voice. Allison reached out to Jack for help, but later in the episode, Amanda was reported as being murdered, with Jack being arrested while covered in her blood.
However, Allison's next dream revealed the truth: Amanda was actually a villainess who had been enlisted by Charles as part of his plan to take full control of Jack, with the added revelation that she intentionally caused the accident and spent time with him as part of the plan. On the night in question, Jack (by his own admission) and Amanda had participated in the drinking game, "Never Have I Ever," and under Charles' order, the evil Amanda asked Jack if he had fantasized about cutting upon young girls and even tasting their blood, with Jack taking a drink because he had done so (the rules of the game). The villainess gave a sadistic smile during the entire event, and afterwards, Amanda stabbed herself to death to set up Jack.