A spellmaster who once made a contract with a demon and is capable of using its power. He's also Oga's homeroom teacher.
Mr. Sadohara, the original teacher, was merely a fill in during the absence of Zenjūrō. Word gets around that Izuma and Tōjō are going to finish the fight they started, on the roof. Oga chooses this moment, the fight unbeknownst to him, to question Izuma about how he knew he had demon powers. All three get their fighting mode on, but are all punched in the face, and subdued by Zenjūrō, who jumps onto the roof from the teacher's lounge.
He is next seen on a roof where a mysterious a dimensional hole has appeared, the conjuror unknown. Zenjūrō punches the hole, revealing his demon tattoos. This act summons strong people acutely aware of the power, and demons. The first to appear was Hilda. A fight ensues, but Zenjūrō keeps insisting that he is not the bad guy, even though Hilda is under the impression he is one of Behemoth's men. Zenjūrō reveals his spell casting abilities to Hilda when he puts her in a binding spell, and leaves. Oga confronts him, demanding to know who he is, to which Zenjūrō replies that he is Oga's homeroom teacher and the latter should be at school tomorrow.