Who is Mrs. Marlin from Mrs. Mike the Story of Katherine Mary Flannigan and what is their importance?

Mrs. Marlin is the woman whose escape from the Territory is the loss of her mind. She plants the garden with no roots as a welcome gift for the Flannigans when they first arrive in Grouard. Later she visits Sarah to get the poison to abort her baby, attempts to steal the Flannigan's son saying he is her child, and admits to killing Cardinal, thus setting Jonathan and Oh-Be-Joyful free.