Mary looks composed at first glance, but the care is almost architectural: blonde hair parted with a ruler-straight line and pinned into a smooth coil, each loose strand tucked away as if disorder might invite something in. Her blue eyes are clear and attentive, with a habit of fixing on a person's mouth rather than their eyes. She moves economically, placing each foot as though testing a floorboard before trusting it. Her breathing is normally quiet and measured; when startled, it catches on a dry, papery click in her throat. Unexpectedly for a miller's wife, she keeps a tiny collection of carefully labeled pressed beetles in a walnut case, fascinated by their armor and patient transformations.

Mary is affectionate and dryly funny with people she trusts, but guarded with anyone who presses for a confession. She is practical enough to dismiss rumors and perceptive enough to notice when the impossible is standing in her kitchen. In a crisis she becomes brisk and capable; afterward, when everyone is safe, she shakes and cannot bear to be touched. She can be tender with Oswin and watchful of him in the same breath.

Mary grew up helping her mother keep accounts and manage the household attached to a riverside mill. She learned to read people by watching what they did when a measure came up short. She married Oswin Hirst after years of friendship and affection, believing his reserve came from grief and long hours. On their wedding night, Oswin woke from a fitful sleep and spoke in a voice that seemed to come from beneath the bed, naming a private childhood promise Mary had never told him. For several minutes his face held the wrong stillness, as if something were learning how to wear it. By morning, Oswin remembered nothing and was tender with her, and Mary chose to believe illness or exhaustion had caused it. Years later, she still loves the considerate, funny man she thinks she married, but she has seen that other stillness return twice. She keeps a lamp burning in the hall and has quietly asked a village physician to examine Oswin, while avoiding any explanation that could make him feel accused. Their marriage remains affectionate in daylight and strained after dark. Mary also sends regular money to her widowed mother, who thinks the Hirst household is entirely happy.