Susan Mailer is the secondary villainess from "Ghosts" episode 1.08 of Castle.
She was portrayed by Jillian Armentante.
Susan Mailer was part of an eco-terrorist group that consisted of Jared Swanstrum and Cynthia Dern, and after the Exxon Valdez Oil Spill, the three of them colluded to plant a bomb on an oil tanker. Swanstrum built the bomb, while Cynthia and Susan planned to plant the bomb.
However, the ship's captain, Sam Pike, got on board after the bomb was placed, causing a snag in the plan. Susan wanted to go back and defuse the bomb, not wanting harm to come to Pike, but Cynthia wanted the plan to go through and escaped while Susan went back. The bomb went off, crippling Pike and (assumingly) killing Susan. In actuality, Susan had survived the bombing, but was wounded and disfigured from the effects. She made a new identity for herself, and she had secretly sent money to Pike and his family every month, doing so out of extreme guilt.
In the episode's events, 20 years after the bombing, Susan came out of hiding when she learned that Cynthia (now going by the name Allison Goldman) was selling her version of the events and writing a novel with Lee Wax. Cynthia's novel depicted Susan as a villainess who insisted on planting the bomb, while Cynthia claimed that she wanted to save Pike and attempted to prevent the "evil" Susan from going through with the bombing. Susan contacted Cynthia and threatened to turn herself in and reveal the whole truth, prompting the villainous Cynthia to plot Susan's murder.
Susan was lured by Cynthia to her hotel room, and attempted to coerce Susan into drinking wine with her, with Susan's glass laced with a sleeping pill. Susan did not drink, however, and she escaped to the bathroom after Cynthia prevented her from leaving. It was there that Susan discovered that the bathtub was filled with motor oil, and she realized Cynthia's true plan: kill Susan and make it look like a suicide committed by a deranged villainess racked with guilt. Susan and Cynthia fought in the bathroom, with the struggle ending with Cynthia being knocked unconscious. Susan later placed Cynthia in the tub and drowned her.
Susan confessed everything to Castle and Beckett, who later arrested her for killing Cynthia.