William is a slight, long-limbed boy whose very black hair grows in a stubborn, forward-falling wedge, as though it is always trying to hide his brow. His vivid blue eyes have a startling ember-bright clarity in lamplight, a likeness to his father Oswin Hirst that people notice before they notice anything else. He smells faintly of chimney soot, damp wool, and the bitter mint leaves he keeps in his pockets; he knows about the mint, but not the soot. He walks with a soft, uneven hitch because he habitually favors the outside edge of his left boot, and can stand so motionless that people sometimes mistake him for being inattentive. The unexpected detail is that he keeps a tiny, beautifully maintained garden of moss and seedlings in broken teacups beneath his bed.

William seems slow to respond, but he is quietly observant and often remembers details adults overlook. He is cautious with strangers and unexpectedly bold when a smaller child needs help. He wants Oswin's approval yet pushes back when his father speaks for him. In a calm mood he can be absorbed and methodical; when frightened he shuts down, then later asks sharp questions about what happened.

William is the young son of Oswin Hirst, and people in their neighborhood call him Black William or Blackby. His unusual black hair and burning blue eyes seem inherited from his father, and older neighbors have begun attaching rumors to the resemblance. When William was five, a kitchen fire trapped him behind a fallen chair while smoke filled the room. Oswin pulled him out, but afterward William stopped answering questions quickly. He learned that adults often decide what a quiet child means before asking him, so now he waits and watches. His mother encourages him to speak up; Oswin tries to protect him by keeping him close, which William finds comforting and suffocating by turns. Their current problem is a string of petty thefts near home. William saw a shadow enter the pantry on the night of the first theft, but cannot tell whether it was a person, a trick of smoke, or one of the strange things the neighbors whisper about.