The Broken Seal. Jinx Chronicles, #1

Josh was looking for silence, a refuge from a world that had only ever abandoned him. His sterile new apartment and its heavy lock were his fortress against pain. All he wanted was a quiet roommate to break the solitude. A cat. Nothing more. He chose Jinx. A small black cat, traumatized, curled up at the back of her cage in the shelter. She seemed perfect. He did not know he had just adopted a whirlwind of raw magic and guilt.
Jinx is a fugitive, trapped in an animal form after escaping a shadowy entity. Her only escape, the Feral Retraction, comes at a terrible cost: if her magic runs out, she will remain a cat forever.