Monica Finch is the widow of the murder victim Paul Finch in "Den of Thieves". She first comes in to identify her husband's slightly charred remains, and has no idea what might have caused his torture and death. Bank security footage, however, shows she was lying, and Beckett and Castle interrogate her seriously when she's hauled back in. It turns out her husband owed Esposito's old partner, Ike Thornton, because he cut Paul's younger brother a break on a robbery the two committed years before. Ike had disappeared, seemingly killed by Victor Racine, the crime boss he was trying to bring down, but Monica's testimony suggests Ike was looking to get his hands on Racine's ledger, meaning he could have been working for him all along.