Akuma

The black cat is not actually a cat — rather, a demon possessing the body of a dead cat. At the lowest point in her life, he convinces Ellen to enter a contract with him, promising a good life and only revealing that she'd become a witch after the fact. After Ellen discovers that her true body is still deteriorating and that her illness hasn't been cured, the demon promises to give her a cure after she feeds him enough human souls. Ellen notes that she doesn't actually trust him, and doesn't answer his attempts to become friends.

The demon is acquainted with another demon who possesses the body of a crow, and brings Ellen medicine on a regular basis.