The plot tells about Rin Okumura and twin brother Yukio, teenagers who possessed abnormal strength and endurance, and were raised by Father Shiro Fujimoto, an Exorcist. Until one day, Rin discovered that he and Yukio are sons of Satan, the strongest of all demons. He witnessed Shiro dying just to protect him and end up doing what Shiro asked him never to do. He draw the demon-slaying blade Kurikara which from scabbard restrains his demonic powers. From that moment on, he gain power to ignite himself with blue flames which can destroy anything they touch, his features permanently changed gaining two horn-like blue flames above his head and blazing around his body, red slit pupils while his irisis remains deep blue, having fangs, pointed elf-like ears, and fur-tufted tail above his hips. As human, Rin has pale skin, dark blue messy hair that sweeps down his face and intense blue eyes, and exhibits physical traits of demonic nature like slightly pointed ears and prominent canine teeth. He wears an open black stiff collared blazer over a white stiff collared long-sleeved polo shirt rolled up to his elbows with stripes black, red and white tie worn loosely, black pants and black shoes.
Yukio, his twin brother, has dark brown hair, blue eyes, and three birthmarks on his face; two under his left eye and one near the right side of his chin. He carry two guns hidden in a briefcase and wear glasses when he is teaching class or reading, and takes it off when goes to bed. His outfit consist of formal uniform when at academy; a buttoned long blazer coat worn as close over a white polo shirt with stripes red, black and white tie, black pants and shoes, and a brown belt with compartments for his weapon.