Mrs. Constance Lenz is Billy and Agnes's mom.
With the help of her lover, she murders Billy's father, Frank, on Christmas Eve 1975 and buries his body in the house's crawlspace. To prevent Billy from talking, she imprisons him in the attic. Years later, Constance attempts to conceive another child, but realizes that her boyfriend is impotent, so she goes to the attic and rapes twelve-year-old Billy. Nine months later, Constance gives birth to their daughter, Agnes.
She also isn't a good mother to Agnes. Years later, on Christmas Eve, her son, Billy, escapes the attic and attacks Agnes, kills her mother's lover, and strangles her with Christmas lights, beats her silly with a kitchen tool, and takes chunks of her skin and eats them like Christmas cookies, for all the times for her horrible parenting.