He is an exorcist since he is 8 years old. His older twinbrother didn't know about demons, until her 15th birthday, and since satan killed their foster father, both fire and flame satan can be defeated. that's why rin is now going to the holy cross academy together with yukio to become an exorcist. Both are satans sons, but only rin are a demon. Yukio is a normal human. Yukio is diligent. smart and wants to be a doctor. he loves and hates rin, and blames rin for the death of their foster father.