Marion Aborn is the woman who impersonates Jane Willoughby.
Times are tough in the 1930's, and Marion is not above rifling through other people's purses looking for loose change. Instead, she finds a letter from Emily Crandall asking Jane to become her guardian and help run the Lilac Inn since her mother Gloria (and Jane's best friend) recently died. Suspecting that she can get some money out of the situation, Marion decides to steal the letter and impersonate Jane Willoughby.
If Emily sells the Lilac Inn before she turns eighteen, her guardian will receive part of the profits. Consequently, Marion does everything she can to make Emily think that she's losing her mind and thus unfit to run the Inn. Discovering a secret passageway, Marion uses it to move pictures, knock books out from their shelves, and whisper to Emily from behind the walls.
After two weeks of this, Emily asks an acquaintance of hers, Nancy Drew, to visit her because she wants Nancy to keep her mother's jewelry in a safe. As the girls talk, Marion sets the kitchen on fire to kill two birds with one stone: stealing Emily's jewelry in all the confusion, and destroying the oven and therefor the Inn's pie business. Without a kitchen, the Inn will be forced to close for months. On top of this, Marion constantly insists that she's no help at all and that Emily must be under a lot of stress. With one misfortune after another, Emily starts to believe that Marion's right.
Later in the day, Marion places one of the stolen necklaces in Emily's hand when she takes a nap. A despairing Emily starts to think that she has only misplaced the jewelry and really is going insane. However, Nancy is getting a little too nosy for Marion's comfort, so she decides to take the rest of the jewelry and skip town.
However, Nancy tracks her down and makes her confess who she really is after seeing a photo of the real Jane.