Juuhime - phantom pain manga

Juuhime - Phantom Pain summary:

From Twisted Mist: A timid boy named Zao Ci, which was accepted by Mateliya, a Demon as her "number 2". He was always called an idiot by Mateliya. He is so weak that he could not even pull the trigger of his gun. But, who actually knows that he had a special power within him, which only Mateliya can control. This begins the adventure of the contract of the boy and his master to save others.