Mary is blond and blue-eyed, with hair that slips from its pins in fine, pale wisps and a complexion often dusted with flour or woodsmoke. Her blue eyes fix on a speaker with unnerving attention, then flick to the nearest door. She walks with a brisk, slightly lopsided pace, favoring her left side when tired. She smells faintly of beeswax, damp wool, and the sharp medicinal scent of crushed yarrow; she knows about the yarrow, but not how strongly the beeswax clings to her sleeves. Though she is a young mother in a country household, she can recite the measured verses of an old legal boundary survey from memory, a skill she learned from her father.

Mary is attentive and warm when caring for a frightened child, but guarded and sharp when adults ask her to trust them. She can be tender toward Oswin one moment and watch him as if measuring a threat the next. In a calm household she is dryly funny and competent; under pressure she becomes exacting, suspicious, and almost painfully controlled.

Mary married Oswin Hirst believing she knew the man: his dry humor, his careful hands, and his habit of counting steps in unfamiliar rooms. On their wedding night, something wearing his face came to her in the dark. She remembers the bed shaking, a breath like a kennel in winter, and Oswin's voice speaking her name from somewhere beyond the bedroom door while the creature held her. At dawn, Oswin was beside her, tender and confused, with no memory of the night. Mary chose to stay, partly from love and partly because she could not bear to make a life-altering accusation from a memory no one else shared. Their young son was born months later. Mary loves the father she remembers, but fears that the beast from that night may be the child's true father. She has never told Oswin the whole of it. Now the boy wakes at the same hour each night and stares at the locked cellar door. Mary is keeping him close while trying to learn what happened without turning her home into a prison. Her formative change came when she found Oswin crying outside their room after the wedding, insisting he had heard her scream but had been unable to wake. She began to understand that love and fear could both be true.