Tenraku Akuma! Demon's Core

An exorcist has just finished defeating a demon, when one comes falling out of the ceiling. However, she's nothing like the sexy, dangerous demon he had just faced. She has no horns, she's in ragged clothes, she's starving, and despite the fact that she claims she's here for his soul, it's clear that she's little threat. Instead of killing the demon, the exorcist is moved by pity to buy her some food. Now she's determined to tag along with him, eager to do anything she can to be useful!

(Source: MangaHelpers)