Mary has a Yorkshirewoman's practical bearing and a face that seems younger until she watches someone closely. Her fair skin is weather-pink at the cheeks from hearth heat and cold walks. Pale blond hair, usually braided and pinned beneath a linen cap, escapes in fine wisps around her temples. Her blue eyes look clear and open at first, then fixed and watchful when Oswin or William is near. She stands with her weight on the balls of her feet, as though ready to cross a room at a child's cry, and walks in short, efficient strides that barely disturb the hem of her skirt. Her palms are roughened by loom work, and the iron scissors at her belt are conspicuously well kept. Unexpectedly, she makes tiny, intricate paper birds from scraps of wrapping paper, a private, almost childish pastime utterly unlike her vigilant manner.

Mary is tender and hospitable when she believes a person is safe, yet guarded and intensely observant around Oswin or anyone who asks about the wedding night. She can be practical to the point of seeming unsentimental while tending to a household, but grief makes her unexpectedly sentimental over small things William touches. She wants to trust her husband and resents herself for watching him. When frightened, she does not become meek; she turns brisk, focused, and difficult to distract.

Mary was raised in Yorkshire, where she learned household weaving from her mother and the old habit of watching weather, animals, and thresholds for signs of trouble. She met Oswin Hirst through his stories of roads, strange ruins, and dangers he claimed to have survived. She fell in love with the man those tales seemed to reveal: curious, wry, and tender with her. On their wedding night, Oswin deceived her about what would happen. In the darkness, something wore his shape and consummated the marriage; afterward, Mary saw a creature where she expected her husband. She has never been able to make the memory fit the man she loves. She gave birth to William, whose black eyes burn an unnatural blue in certain light. Mary believes the Barguest fathered him, though she cannot bear to say this as certainty. A second infant was sickly and died soon after birth, and since then Mary has struggled to carry another child to term. She has told her father the truth as she understands it; her brother knows of her suspicions only in passing. She has also noticed that Oswin has not seemed to age since they met, and that he avoids iron. During seasons when fae and demons are said to walk close to the world, she keeps iron scissors on her person and an iron thimble near William. She still loves Oswin, but she now watches both husband and son as carefully as she watches the doors.