Jem is a thin young man whose narrow shoulders seem to fold inward whenever a room grows loud. Dark circles shadow his eyes, and a faint sheen of machine oil makes his fingertips look permanently varnished. He moves in quick, abbreviated starts, as if every step is a correction to a measurement only he can see. When rattled, he takes tiny sips of air through his nose; when focused, he lets out a long, audible breath that fogs his brass-lensed magnifier. Unexpectedly, he keeps a pocketful of sugared fennel seeds and offers them to anyone who looks frightened, an oddly tender habit for a workshop apprentice who usually treats people like complicated mechanisms.

Jem is cautious with people but reckless with machines, and he is proud of solving problems yet embarrassed by praise. In a crisis he becomes focused and brisk; in an ordinary conversation he second-guesses every joke. He resents being treated as a child, but secretly seeks Orest's approval. His compassion is practical: he may not know what to say to a grieving person, but he will mend their stove before nightfall.

Jem grew up above a repair shop called Sootbell, named for the clanging warning bell and the smell of its coal stove. His mother, Dena, repaired kitchen clocks and door latches; his father left when Jem was small, leaving behind a broken brass astrolabe Jem has never managed to mend. At thirteen, Jem was apprenticed to Master Orest, a precise, impatient craftworker who taught him to listen to gears before touching them. During a winter fire in the shop, Jem froze at the sound of the bell and failed to release a jammed rear shutter. Orest dragged him clear, but a neighbor's workshop was lost. Jem has since trained himself to act quickly in emergencies, though the old bell still makes his breath catch. He now works to build a cheap warning device that will alert the whole block before another fire spreads. His mother wants him to stop taking dangerous commissions, while Orest quietly fears Jem will leave before learning the craft properly. A young courier, Pella, brings him broken trinkets and news from the streets, and Jem repairs the trinkets without charge.