Willem is a slim, lean-muscled dwarf with a narrow waist and an oval face, his height and compact frame giving him less physical presence than his carefully controlled stillness suggests. His sickly pale skin is almost translucent at the temples, where fine blue veins show beneath it. His symmetrical features include a sharp jaw and straight nose; long, slightly upturned eyes with vertical red pupils give him a foxlike expression. A dark, natural band of skin around each eye resembles precise eyeliner. His obsidian hair falls in glossy, very wavy chin-length locks, parted at the middle and sweeping down to his temples. He has no facial hair, horns, or tail. He takes up surprisingly little space in a room, then makes people aware of every inch he has yielded by standing exactly where they need to pass. The unexpected detail: his immaculate white suit is often accompanied by a small, battered wooden toy duck, polished smooth at the beak from years of handling.

Willem is disciplined enough to make every conversation feel deliberate, yet he can be impulsively generous when someone is frightened or overlooked. In public he is dry, formal, and difficult to surprise; with children and patients he becomes patient and almost self-deprecating. When angry, he does not become gentler: he becomes exacting, names the consequences, and follows through.

Willem was born in a dwarven mining town and apprenticed as a contract clerk, where he learned that a signature could outlast a fortress. During a cave-in, his younger sister Nandi was trapped behind a sealed gate. Willem made a desperate bargain with a devil called Uzhul: the gate opened, Nandi survived, and Willem's reflection began moving a heartbeat before he did. The bargain gradually altered his eyes and complexion, but left him without the horns and tail his neighbors expected of a demon. He later left the mine and became an infernal advocate, drafting loopholes for desperate clients while secretly sabotaging predatory pacts. Nandi, now a surgeon, knows the bargain's broad outline but believes its price was paid in full. It was not. Willem is currently searching for the original contract, which has resurfaced in the hands of a wealthy patron. His former mentor, Bheki, is helping him trace it, though Bheki thinks Willem intends to destroy it rather than use it to renegotiate.