A petite, slight young woman who stands with the loose balance of someone accustomed to wet ledges. Her warm ivory skin is pink at the nose and knuckles from the cold. Damp dark curls spring around her jaw, and her bright hazel eyes keep flicking to rooflines, drains, and the high corners of a room before settling on a face. Her hands are callused across the fingertips and faintly ink-stained at the side of one thumb. She moves lightly, placing each boot as though testing a roof tile, and leaves an impression of alertness held just short of flight. Unexpectedly for a nimble rooftop runner, she carries a little wooden pot of rooftop-garden mint and worries over its leaves when she thinks no one is looking.

Ylva is quick to help but slow to trust, and her practical kindness can look like suspicion until someone needs a hand. She is bold on a roof where she knows every foothold, yet guarded in a comfortable room where she cannot see the exits. In a crisis she becomes crisp and decisive; after it passes, she may shake with delayed fear and make a joke before anyone can ask how she feels.

Ylva grew up in the narrow stair district above the old canal, where her mother, Ragna, repaired roof drains and her father vanished on a winter barge when Ylva was six. As a child she learned the roofs by following water: where it gathered, where it ran, and which gutters sang before a storm. At fourteen, she carried a warning across the rooftops during a sudden flood, but arrived too late to save the rooms below her neighbor Jarek's stair. The event changed her from a reckless climber into a careful observer who maps exits before she enters a place. She now works as a courier and occasional finder of lost people, passing useful routes to residents who cannot afford the official watch. She remains close to Ragna, though they quarrel over Ylva's habit of taking dangerous jobs. Her former running partner, Borys, has started selling route information to a property factor who wants poor tenants pushed out. Ylva is trying to learn what Borys has promised without turning him over to the watch.