A very pale, slight boy with a watchful, solemn face and a fall of dense black hair that sticks up at the crown no matter how often it is combed. His vivid blue eyes, inherited from Oswin, seem startlingly bright against his colorless skin. He stands with his weight gathered over the balls of his feet, as though ready to follow whoever has earned his trust. When he laughs, his small jaw opens wide and shows a gap where a front tooth has recently fallen out. He moves carefully, pausing to study thresholds and faces before crossing a room. For a child expected to inherit a household, he has an unexpected fondness for muddying his hems by crouching to inspect beetles and worms.

William is cautious with strangers but intensely curious about small, concrete things. He may seem withdrawn when adults press him to perform, yet becomes animated and persistent when he can show someone a beetle, a pattern in the floor, or a familiar toy. He wants his parents' approval, but resists being hurried, often by going quiet rather than openly refusing.

Mary Hirst's labor was long, exhausting, and frightening. When William was born with black hair and Oswin's burning blue eyes, Oswin took him in both hands with a care so grave that the midwife later remembered it more clearly than the storm outside. William was named for Oswin's family, but the village soon called him Black William, or Blackby, for his hair. He has been slower than other children to speak, read faces, and master lessons expected of an heir. His maternal grandparents and his uncles Edmund and Huge fear that he may not be able to inherit or manage the Hirst household; their worry comes out in very different ways, from anxious instruction to whispered contingency plans. Mary tries to protect William without making him feel watched, while Oswin's solemn tenderness can make every stumble feel like a test. William's own world is smaller and more vivid: he knows the sound of his mother's steps, the shape of his father's hands, and which garden stones shelter beetles. His current problem is that the adults have begun discussing his future as if he were not in the room.