A small boy with a serious, almost watchful face, William has dense black hair that stands in uneven tufts as if it has been rubbed dry with a towel. His vivid blue eyes have an unnerving, ember-bright clarity in lamplight. He walks with a slight inward turn of his toes and stops abruptly when something catches his attention, leaving the impression that his thoughts have wandered away from the room. His hands are usually smudged with charcoal or flour from tracing shapes on whatever surface is nearby. Unexpectedly, he keeps a little bundle of dried meadow flowers tucked into his coat and carefully replaces any that crumble.

William can seem slow to answer, yet his attention is sharp and stubborn once something catches it. He is gentle with frightened animals and unexpectedly firm when an adult dismisses Mary. In unfamiliar company he retreats into silence; at home, when relaxed, he can become a relentless questioner. He wants to be brave but is still a child, and fear sometimes makes him freeze instead of act.

William Hirst is the young son of Mary Hirst, a connection of Oswin Hirst. From infancy he was conspicuous: black hair where the family expected brown, and blue eyes so bright that neighbors began calling him Black William, or Blackby. The name stuck, though Mary dislikes hearing strangers use it. When William was small, he became lost for an afternoon during a flood evacuation. He was found sitting on a dry stone step, calmly sorting soaked buttons by size while adults searched the lanes. Since then, people have mistaken his long pauses and intense concentration for slowness. Mary knows he notices more than he lets on, but she worries that he cannot always explain what he understands. Oswin's resemblance is a source of family speculation, never an answer: William has inherited the hair and eyes, but no one agrees what else that might mean. Mary works hard to keep him out of local gossip while balancing the ordinary troubles of raising a child with little money and too much attention. William has begun slipping away to the old flood steps, where he says the stones remember voices.