Mary has clear blue eyes and blonde hair usually pinned into a practical coil, though loose strands curl damply at her temples in the Hirst house's persistent chill. She stands with her weight pressed forward onto the balls of her feet, as if prepared to cross a room or flee it in the same instant. When she waits, she angles herself toward the nearest door without seeming to notice. Her square jaw works visibly when she eats; her neat, broad front teeth show in a startlingly unguarded laugh that disappears as soon as she remembers to be afraid. She leaves the impression of someone holding a conversation with one ear turned toward the next room.

Mary is compassionate enough to sit through another person's grief without trying to mend it, but guarded enough to notice what they do not say. In a calm household she is brisk, wry, and generous with practical help. When Oswin is involved, warmth and suspicion tug against each other: she may defend his best qualities in one breath and quietly test his memory in the next. When frightened she grows precise rather than tearful, turning feelings into lists and observations.

Mary grew up as the practical daughter of a village midwife, learning to notice what people omit while they describe what hurts. She married Oswin Hirst after a patient courtship that made her believe she had found a kind, steady man beneath his reserve. On their wedding night, Oswin woke speaking in a voice Mary had never heard, stood unnaturally still in the moonlight, and addressed her by a name from a language neither of them knew. By dawn he was himself again, frightened and ashamed, and begged her to keep silent. Mary chose to believe his terror was real, but the memory never settled into a dream. In the years since, Oswin has had blank spells and unexplained nights away; Mary quietly records dates, weather, and the odd phrases he mutters in his sleep. She still loves the thoughtful man who brings her tea and remembers the names of her mother's patients. She fears the other presence that sometimes looks out through his eyes. Their marriage is strained by her careful questions and his evasions, while a nearby hamlet's recent reports of livestock found standing in perfect circles have made her wonder whether the presence is growing bolder.