Kathy Willowby (Faith Prince) is a villainess from "Mr. Monk Gets Cabin Fever," episode 3.12 of Monk (airdate February 4, 2005). She 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 villainous 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, knowing that it annoys Martin. At that precise moment, the evil Kathy tossed the radio into the tub, and gave an evil laugh 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 bullets shot by members of a Chinese gang, and Kathy is arrested.
- "Mr. Monk Gets Cabin Fever" is the only Monk episode to feature two villainesses affecting different characters, as the episode also featured Hayley in the subplot featuring Randy Disher.