Abby Brewster is a fictional character from the play "Arsenic and Old Lace," written by Joseph Kesselring. She is portrayed as a sweet, elderly woman who runs a boarding house with her sister, Martha Brewster. Abby is known for her charming demeanor and her seemingly innocent nature. However, beneath her kind exterior, Abby has a dark secret. She and Martha have been poisoning lonely old men as a form of mercy killing, believing they are helping them. This shocking twist adds a layer of dark comedy to the play, highlighting the contrast between Abby's appearance and her actions.