Mary has the tired, watchful look of someone who has learned to keep one ear on the stove and another on the children. Her dark blond hair is usually pinned back with whatever is at hand, escaping in fine wisps around her temples. Her blue-grey eyes seem pale in lamplight and almost steel-colored outdoors. She moves quickly but quietly, placing her feet as if a sleeping child might be just around the corner. Her jaw is sturdy and slightly prominent; when she laughs, she shows a small gap between her upper front teeth and an unexpectedly broad, unguarded grin. The oddest detail is that she instinctively checks her reflection for the young man before she checks her own face.

Mary is kind without being soft-headed about hunger, work, or danger, yet she remains strangely naive about how readily adults exploit affection. She can be patient with a child's mistake and merciless toward an adult who frightens one. In a crisis she is practical and almost startlingly brave; afterward she may go quiet, ashamed of how scared she was. She resents being treated as a second mother but cannot stop acting like one.

Mary Bell grew up in an isolated house where her parents' arguments were louder than any lesson and the children learned to fend for one another. As the oldest of eight, she became cook, nurse, peacekeeper, and reluctant substitute parent while still trying to school herself. Simon Bell was often away working, while Malinda Bell drank too much and could be careless with the children. Mary loves her siblings fiercely, especially Matilda, whom she refuses to let be turned into Mary's replacement, and little Owen, who follows her with his blanket when he can. Henry's resentment has made him sharper and more troublesome lately, and Mary fears she is losing him to the same anger that poisoned their parents' home. On Mary's eighteenth birthday, Malinda brought her a curious stone found lodged in a log. Mary accepted it because refusing would have hurt her mother, though she thought it an odd and unimpressive gift. Later, the stone began beating in time with Mary's heart. Her reflection changed into that of a young man with short bright blond hair and clear blue eyes. She tried to ignore it while taking part-time work to help feed the family. The day the reflection spoke, it apologized in a voice that was not hers, then awkwardly shuffled away from its own reflected position. Since then, Mary has heard it only intermittently. She has not decided whether it is a prisoner, a parasite, or another frightened person. The family remains in their isolated house in Cold War England. Mary works when she can, keeps the stone secret, and tries to protect her siblings without making them feel like burdens. The young man in the stone appears to know fragments of the world outside the glass, but neither of them trusts the other's account of how he came to be there.