Marshall Sim (played by Rob Freeman) was a man who married Roberta Sim and was the adoptive father of Emily Christine Sim.

Sim and his wife took Emily to see Doctor Ernest Calderon to participate in a double-blind study for her illness. Roberta did not like the treatments, and, as a compromise, the payments for the program were given to her directly. When she still filed paperwork to take Emily out of the program, Sim pulled the papers back. This caused an argument which was heard by neighbors and a domestic violence report was filed.

While Sim and Emily were at an appointment, Roberta was killed in a manner that looked like suicide. Detective John Kresge led the San Diego police force handling the investigation. He concluded it was suicide, but, with the assistance from Dana Scully, it turned into a murder investigation, to Sim's frustration. Two hybrid agents, who Sim knew as from the pharmaceutical company, came to his house. Sim's alibi held up but he was still incarcerated in the local jail. The agents discretely made it seem that Sim confessed to the crime and killed himself in his jail cell. Scully soon found the body after seeing the agents leave the police building. She and Kresge then found Sim dead in his cell, apparently killed by hanging himself from the upper bunk. Mulder agreed with Scully that both his death and the death of his wife were undoubtedly the work of Syndicate agents out to eliminate the threat posed by the existence of Emily. (TXF: "Christmas Carol")

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