Zenjūrō Saotome

Zenjūrō 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 Tojo in a matter of seconds. Also, he appears to be both Tojo and Shizuka's mentor, and a spell master from the Demon World.

Appearance
Zenjuro has long shaggy black hair, and a goatee and mustache. He sports a thick bandana with beading on it, which covers all of his forehead. He has the marks of a powerful demon tattooed all over his arm, and has the seal of the royal crest on his hand. He is often seen smoking.

Personality
Zenjuro is somewhat of a pervert: he "especially likes the uniforms" of Japanese schools, and tells Kuneida outright that he thinks she's cute, and wonders if she has a boyfriend. When Hilda and Zenjuro start fighting, he gropes her breasts, and comments on her lingerie. His demeanor is casual, but intimidating because of his size and power. He has taken up an interest in Oga and Beel. He appears to know Kunieda's Grandfather somewhat.

Plot
Saotome Zenjuro makes his first appearance quite early on in the manga, in a flashback of Tojo's in Chapter 37 of the manga. Tojo tells Oga how he looked up to Zenjuro, and ever since he first saw him when he was about 12 years old, he's always aspired to be as strong as him. Tojo compares Zenjuro and Oga, and realizes that Oga is "the real thing"- Ishiyama's best, not purely because of strength, but also because of what he was protecting, in the words of Zenjuro. He reappears briefly in Chapter 76 to comment on the ruckus Ishiyama High students are making at St. Ishiyama's, after the fight that ensues following the volleyball match.

Saotome Zenjuro becomes the homeroom teacher for the Ishiyama delinquent class- the actual assigned teacher. Mr. Sadohara, the original teacher, was merely a fill in during the absence of Saotome. Word gets around that Izuma and Tojo 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 Saotome, 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. Saotome 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 Saotome keeps insisting that he is not the bad guy, even though Hilda is under the impression he is one of Behemoth's men. Saotome 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 Saotome replies that he is Oga's homeroom teacher and the latter should be at school tomorrow.