Susan Plummer (died 2018) was an American DIA agent and security consultant who lived in Boston, Massachusetts during the 2010s. She worked with Robert McCall until McCall's supposed death in a car bombing, although Susan doubted the reality of his death. Plummer went on to marry Brian Plummer and live with him in a rural mansion, where McCall visited her in 2014 and enlisted her help in his fight against the Pushkin crime family. She provided him with the identities of his rivals within the Pushkin family and their police connections, and she encouraged him to pursue his righteous quest after hearing of his reason for it, defending the teenage prostitute Alina Yesipova.
Four years later, Plummer continued to assist McCall with his vigilante activities, including rescuing librarian Grace Braelick's daughter from an abductor in Istanbul. Plummer and fellow DIA agent Dave York investigated the murder-suicide of a DIA affiliate in Brussels; however, when she went to her hotel room, she was murdered by several criminals hired by York to cover up the murder-suicide. McCall soon discovered this, eventually killing York and avenging Plummer's death.