Tessa is a short, slight young woman whose compact frame seems made for slipping past elbows in a crowded lane. Her skin has the soft, lightly flushed look of someone who spends long hours outdoors but avoids direct sun. Her dark brown hair is cut just below the jaw and tucked behind rounded ears, though damp curls keep escaping at her temples. Bright hazel eyes flick from drain mouths to rooflines before settling on a face. When she laughs, her broad front teeth show in a quick, surprisingly unguarded grin; when she eats, she bites with the decisive focus of someone accustomed to taking a meal between errands. She stands comfortably with one foot braced near a railing, then moves in short, careful steps, testing slick stone before committing her weight. The impression she leaves is of a sparrow that has memorized every way out.

Tessa is quick to help and slow to trust, generous with practical advice but evasive about her own needs. In ordinary company she uses jokes and brisk competence to keep attention moving elsewhere. When a person is in immediate danger, the awkwardness falls away and she becomes sharply commanding. When she is alone, she can spend an hour worrying over a choice she made in a few seconds.

Tessa grew up above a tallow shop in the old quarter, where her mother, Irena, repaired roof gutters and her father, Davor, carried messages between workshops. At twelve, Tessa began running parcels along the rooftops because the streets were often flooded and the stairs were quicker if you knew which ones held. During the Great Thaw, a blocked drain burst through a tenement wall. Tessa heard a child trapped behind the waterlogged plaster and led neighbors through a maintenance crawlspace to reach him. The rescue worked, but the flood also exposed a sealed arch beneath the building, one that city surveyors had been told did not exist. Since then she has mapped the old drains, partly from duty and partly from the uneasy certainty that the city has been built over things its owners would rather forget. Her mother now depends on her to find steady work; her father left the city years ago and sends brief letters from distant road crews. Tessa is fond of Boro, an aging lamplighter who taught her which rooftops are safe, but they quarrel because he thinks her curiosity will get her killed. Her current problem is a series of fresh mortar plugs appearing in passages she knows are open, while tenants nearby report a faint knocking beneath their floors.