Kathy Willowby is a one-shot villainess from Monk, appearing in Season Three's "Mr. Monk Gets Cabin Fever."
She was played by Faith Prince.
Kathy Willowby is the wife of Martin Willowby, and the couple lived at a cabin near where Monk, Natalie, and Stottlemeyer are holed up; due to Monk being in witness protection. Monk and Natalie meet the couple at a local shop, where Kathy plays country music knowing that Martin hates it.
Kathy's evil nature is revealed later in the episode, as she had been wanting to kill Martin to inherit his insurance money. While Martin was bathing, Kathy went on about how she would spend the money, stating that she would go on a trip to Hawaii. She later turned on the radio, which played country music, much to the annoyance of Martin, who told her to turn the radio down. At that moment, the evil Kathy tossed the radio into the tub, and laughed evilly as she watched her husband get electrocuted to death. After killing Martin, Kathy placed his body on ice and then put him on his fishing boat and cut it loose, because she needed Martin's death to be caused by an act of God to inherit the insurance money. Kathy's evil plan is figured out by Monk, while he and the others are dodging bulliets shot by members of a Chinese gang, and Kathy is arrested.