The Devil's Secretary

Catherine has been the Devil's secretary for centuries. She has handled any appointments the Devil may have for making or upholding deals. All was as normal as life could be.... that was, until a new generation of Hunters made they're presence known.
Hunters have been around almost as long as the Demons have. They made it their goal to wipe out as many demons as possible. Up until now, it had been next to impossible to kill a demon, but with advancement in technology, the task has become worryingly less of a hassle.
With more and more of Satan's demons dying, he is forced to recruit new forces from anywhere he can get them. Including in those brave and stupid enough to have made deals with him.
Next thing she knows, Catherine is being sent out into the mortal realm to collect souls to add to the forces of the underworld. On one such mission, she finds a surprisingly compliant boy, whom she later learns is hers to train....

(Sorry if that description sucked. I'll probably fix it another day. Also, if anyone wants to help with a better cover, please, please, PLEASE do?)