Aki is a Devil Hunter who is left in charge of a young girl who grows by his side as the years go by. He protects, educates and raises her as if she were his own little sister. Aki is a good guardian even though he is someone with a serious and repetitive personality, he loves her unconditionally. Even though she has grown up, Aki still sees her as his little girl. He is somewhat depressed after the death of his other friend and now he thinks that his little sister is hiding things from him.