Dead Weight - Mary Robinette Kowal

Unnur was living a normal, if lonely, life until a black cat showed up at her door. Trying to do the right thing, Unnur reunites the lost pet with its owner-a young woman named Asta who is in desperate need of some help. Unnur reluctantly agrees to take in the cat until Asta is able to care for it again herself. Soon, Asta becomes a fixture in Unnur's life and the two form an unlikely friendship. But like a black cat, trouble is tailing Asta, and Unnur is the only one there when things take a violent turn. Nothing tests a friendship like blood on your hands.