Eric Marx is the son-in-law, and co-conspirator in the murder, of Frank Anderson, seemingly over a matter of a loan and a rent-controlled apartment. He first appears in "The Double Down" as a supportive husband, helping his wife, Christina Marx, cope with the loss of her father. When Beckett and Castle piece together the actual chain of events which make up this latest case, she instantly hones in on Eric as the secondary, and thus submissive, in the plot.