Thessa has a compact, sturdy frame and moves with the brisk, sidelong precision of someone used to slipping between carts and people. Her dark curls are chopped unevenly at her jaw, usually flattened on one side by a knitted cap. She wears her sleeves rolled to precisely the same height, though one cuff is patched with a square of faded sailcloth. Her most unexpected detail is a tiny brass bell sewn inside her collar, not for luck but so she can hear whether a baby she is carrying is still breathing. She leaves the impression of someone already halfway to the next urgent task.

Thessa is generous in tangible ways but guarded about tenderness. She admires courage and distrusts the way courage can become an excuse to ignore limits. In a crisis she is brisk, clear-headed, and unexpectedly commanding; at rest she is funny and observant, but becomes prickly if someone praises her for sacrifices she never agreed to make.

Thessa and Brenna Lethwyn share a mother but grew up in different households. As a child, Thessa idolized Brenna's habit of stepping toward trouble when everyone else stepped away. That admiration changed during the winter flood at Reedcut, when Thessa was thirteen. Brenna left their mother's sickbed to help strangers trapped in a flooded tannery; she returned safely, but their mother died before dawn. Thessa knows the choice was not simple and has never accused Brenna of causing the death, yet the memory made her wary of treating every crisis as a summons. She now works as a laundress and part-time birth attendant near the river market, where she has built a web of practical favors with carters, midwives, and stallholders. She sends Brenna notes about their mother's belongings and about the rent arrears on the room they inherited, but their exchanges keep circling the same argument: Thessa wants her sister to choose when to help, rather than surrendering every claim on her own life.