Mary has the wind-roughed look of someone who works near a draughty Yorkshire hearth: fair skin pinked at the cheeks, faint freckles over her nose, and a pale line at one wrist where a loom shuttle often rubs. Her blond hair is usually twisted into a practical knot, though loose wisps cling to her temples when she works. She stands with her weight slightly forward, as if ready to cross a room at the first sound from William's bed. When unsettled, she goes very still and watches the empty space around a person rather than their face. An unexpected detail for a weaver and homemaker: she can identify many local birds by their tracks in mud, a skill learned while searching the fields for her wandering brother as a child.

Mary is affectionate and practical, but her warmth does not make her credulous. She can laugh easily with Oswin and then spend the night watching his shadow for signs of something wrong. In calm moments she is patient, even playful; when William is threatened she becomes brisk and immovable. She wants to trust her husband, yet keeps iron close and remembers every evasive answer.

Mary grew up in a Yorkshire cottage where every scrap of cloth was mended and every child learned to watch the weather. She married Oswin Hirst for his stories as much as his steadiness. He spoke of roads, strange courts, and adventures that sounded impossible but left her laughing beside the fire. On their wedding night, however, the man she expected was replaced by a headless shape with red, saucer-like eyes burning in the space where its head should have been. Oswin later returned to his familiar self, loving and attentive, but would not give her a truthful account of what happened. Mary remembers the creature and still loves the man she knew before it appeared. After William was born with black hair unlike either blond parent, and vivid blue eyes like Oswin's, Mary began to believe the child might be the Barguest's. William is slower to speak and learn than other children his age, and Mary is fiercely patient with him. A later baby died soon after birth, sickly and small; since then Mary has struggled to carry another child to term. She watches Oswin and William especially when the seasons make the boundaries thin. Oswin sometimes guards the house through those nights, and his eyes can gleam red in the dark, though he still makes time for his wife and son. Mary has told her father what she remembers, and her brother knows a little. Her father, who lives a good distance away, is already considering how Oswin's secrets might benefit the family. Mary has also noticed that Oswin does not seem to age and avoids iron.