Juuhime - Phantom Pain

From Twisted Mist: A timid boy named Zao Ci was accepted by the Demon Mateliya as her "number 2". He was always called an idiot by Mateliya. He is so weak that he cannot even pull the trigger of his gun. But what no one knew was that he had a special power within him which only Mateliya can control. Thus begins the adventure of the contract of the boy and his master to save others.