The Pain Eater by Kyle Muntz PB

The Pain Eater is the story of two brothers from Michigan reunited after the death of their father.

They've never been close, but now they have to live together--and it gets more difficult when one discovers a strange creature, vomited from the body of a dead cat. A creature that eats human pain. It feels good: too good. Soon he wants to hurt himself more, just so the pain can be taken away. But the more the creature becomes a part of his life, the more he damages everything around him. Some wounds are too deep to ever heal.