Marilyn was a KGB agent who initially just ran surveillance for Philip and Elizabeth on their missions, often with her partner Norm. She was recruited by Gabriel, who said "You wouldn't even notice her if she sat on your lap." Marilyn worked as a librarian; Gabriel approached her after noticing she had a copy of "The Militant" in her bag.
After Philip retired from Directorate S, Marilyn, Norm and Paige became Elizabeth's main partners. Marilyn knew Paige by the name of Julie, and remained unaware of Paige and Elizabeth's connection to one another.