Mrs. Willow is the ex-wife of Chase B. Willow.
Mrs. Willow was a locksmith and the wife of Chase B. Willow. However, she wanted to leave her husband, so she concocted a plan to get him arrested so she could run away with a lawn mower technician.
When Chase competed in the Ethan Frome Race, Mrs. Willow hid amongst the sledders with her own, unmarked sled, concealing the stolen painting of Gary Dorian, Stain'd-by-the-Sea's famous cosmetician. Unbeknownst to her, he was photographed with the unmarked sled.
She hid the painting in the attic. Her husband invited the town to celebrate his win at his house, and during a game of hide-and-seek, the Officers Durham found the painting left in plain sight.
Just as planned, her husband was arrested and charged with the crime. Mrs. Willow divorced him while he was in jail and moved in with the lawn mower technician.
It is possible that she felt guilt over this, as her ex-husband is noted to have escaped prison with a skeleton key that she, as a locksmith, may have smuggled him.