Zenjuuro Saotome

Zenjuuro Saotome is introduced as a "friend" of the principal. He graduated from Ishiyama High, the delinquent high school the main characters used to attend, and is now apparently a formidable teacher. He demonstrates formidable strength when he beat Oga, Izuma, and Toujo in a matter of seconds. Also, he appears to be both Toujo and Shizuka's mentor, and a spell master from the Demon World. Zenjuuro is the most powerful human described so far, and one of the most powerful characters in the series shown so far. He shows superhuman strength, speed and agility and is able to defeat uncontracted demons with relative ease, including Hilda, as well as being able to defeat Toujo, Oga, and Izuma with little effort.

Even Behemoth has expressed that he would rather not fight him if he could avoid it due to his strength. It is implied that he has access to information from the demon world that even Hilda is not privy to. His control of his demonic powers is much greater than Oga's, and he can gauge the amount of energy he wants to use with precision, as seen by the fine control he has over the size and expansion of his own tattoo. Recently, he offered to teach Toujo how to fight a demon. It seems that he is acting on the headmaster's orders: in the school, he is appointed as homeroom teacher to the Ishiyama students.