She was played by Julie Bowen.
Patrice Gesner is the long-lost twin sister of Marilyn Brody (also played by Julie Bowen), but the sisters were separated 35 years prior and adopted by different families. She learned about her sister from Douglas Fendle, a lawyer who handled their adoption, and after learning about Marilyn's wealth, the pair worked together to get Marilyn's finances. Not wanting to share the wealth, however, Patrice killed Fendle with a steak knife, and then went to the Marilyn's home with intent on switching places with her.
Patrice sprayed Marilyn with mace and then tied her up and locked her in the downstairs attic. Patrice's evil plan also included killing Marilyn's husband, Roderick Brody (Monk's school bully), but instead, he was arrested for Fendle's murder. When learning this on the phone, Patrice entered the police station as Marilyn and stated that she was in love with Fendle, and claimed that Roderick went into a rage when she told him about their affair. She also falsely claimed that Roderick was threatening to kill Fendle, and gave Stottlemeyer the steak knife, claming that Roderick told him to hide it.
With Roderick out of the way, the evil Patrice worked on her final piece of her plan: killing Marilyn. She took Marilyn to her bathroom and filled up her bathtub with sea salt; planning to drown her in the tub and making it look like she killed herself at sea out of depression. While this was going on, Monk was at the door with Natalie, Stottlemeyer, and Randy, stating that he figured out that Marilyn had a twin due to little differences: the different pronunciation of "aunt," and the fact that Marilyn had bitten her nails, while Patrice's were perfectly manicured. Once Marilyn's screams for help were heard by the quartet, they rushed in and subdued Patrice, who was handcuffed and arrested.