A small, watchful boy with a soot-dark thatch of hair, a narrow chin, and a solemn face that can break into a startlingly broad grin. He walks with his toes turned slightly inward, quickening into a sideways scuttle whenever he thinks someone is watching. His wool coat is too large and its left cuff is polished smooth from being worried between thumb and forefinger. He leaves the impression of a child who is listening to a conversation happening one room away.

William wants to be included in adult affairs but is quick to retreat when attention turns serious. He is mischievous with children and solemn with strangers; when relaxed, he chatters and invents elaborate explanations for ordinary things. When frightened, he becomes practical and watchful rather than tearful, then shakes afterward once he is safe.

William Hirst is the young son of Oswin Hirst's connection, raised in a cramped house near the old river road. His family call him Blackby; strangers sometimes use Black William, usually after noticing his ink-dark hair and the way he watches them. When he was five, a winter flood trapped him in a loft with a frightened neighbor's infant while the adults tried to reach them. William kept the baby warm and quiet by telling it the same nonsense story for hours. Since then, he has been both more protective of small things and less willing to believe that grown-ups will arrive in time. Oswin has visited often enough to become a familiar, complicated presence: William likes his attention, but has begun to suspect that adults speak around him when Oswin is involved. His mother, whose name he keeps out of strangers' mouths, is working double shifts at a riverside bakehouse and is behind on rent. A landlord's agent has begun asking pointed questions about the family's connection to Oswin.