The girl had lost her sister before the book and her family spent a lot of money trying to find her, making them move to a trailer park to still have a place to stay. The girl meets Jack, a beat up cat with one eye, and he sees a chance in something with Emma. He gives her a deal that states if he helps her find her sister, she helps him.
So far in the book, I'm about half-way through. I won't give any details, but she is definitely the heroine of this story. She also obtains cat-magic, a special kind of magic that actually does something. Not like faery magic that only tricks someone into seeing it. Cat magic turns you into something.