A small child with an intent, almost solemn face and a tendency to stand very still while other people talk. His black hair is thick and unruly, with a blue sheen where lamplight catches it; it curls at the nape as though it has been dampened and left to dry. His eyes are an unnerving, vivid blue, bright as heated glass, and seem to hold a pinprick of light even in a dim room. He moves cautiously, placing each foot with care, then suddenly hurries after something that has caught his attention. He smells distinctly of woodsmoke and crushed mint, a trace from the little bundles Mary tucks into his coat. William is unaware of it. The unexpected detail is that he patiently mends small tears in other people's clothes with clumsy, oversized stitches, even when nobody asked.

William is observant but slow to assemble his observations into an answer, patient with small tasks but easily bewildered by hurried instructions. He is shy with strangers and unexpectedly firm when he thinks someone is being treated unfairly. When safe, he can be quietly funny; when frightened, he retreats into silence rather than crying out. He adores Oswin but senses his father's fear, and sometimes acts brave to spare him worry.

William is the young son of Oswin Hirst and Mary Hirst, herself a connection of Oswin's. Mary has raised him among the close-set cottages of Survey Row Nine, where everyone knows when a child has a fever and who has failed to bring home the bread. Oswin dotes on William, teaching him small practical things and making up excuses to carry him on his shoulders, but his affection is tangled with fear: William's unusual hair and burning blue eyes resemble his own, and the same strange signs that once marked Oswin have begun to appear in the boy. William can seem slow to understand a question, especially when several people speak at once. Mary says he needs time, not scolding. When William was five, a frightened draft animal bolted toward a market cart. He froze, then stared at it with those bright eyes; the animal stopped as if it had heard a command. Oswin saw the same uncanny stillness he remembers from his own childhood. Since then, he has tried to keep William away from crowds and anyone who might take an interest in the boy. Mary resents being treated as though hiding their son is the same as keeping him safe. Their current problem is practical as well as frightening: a local reeve has demanded that William be brought to a spring census after reports of blue lights near the old well. Mary wants to go openly; Oswin wants to leave before dawn.