Who is Maggie McDonald from Mercy and what is their importance?

Maggie McDonald is Jamie's wife. While ice-skating a year or so before the opening of the novel, Maggie breaks her ankle in a fall. Jamie takes her to the hospital where an x-ray shows evidence of bone cancer. Soon after, Maggie and Jamie learn that Maggie has breast cancer that has spread to her bones. Maggie undergoes treatment for the cancer that includes both surgery and chemotherapy, but the cancer continues to spread. When the cancer spreads to the optic nerve, Maggie becomes depressed and begins to ask Jamie to kill her.