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 Tōjō in a matter of seconds. Also, he appears to be both Tōjō and Shizuka's mentor, and a spell master from the Demon World. Saotome, Genma Isurugi and Ittosai Kunieda are somehow connected.

Appearance
Zenjūrō is a fairly tall adult possessing long shaggy black hair, as well as a hair stubble growing around his face and upper lip. He sports a thick bandanna with beading on it, which covers all of his forehead. He has the marks of a powerful Demon tattooed all over his arm, as well as the seal of the Royal Crest on his hand. Zenjūrō is often seen smoking.

As a teenager at Ishiyama High School, Zenjūrō had a somewhat different appearance. His hair was shorter and straighter, with his bangs falling over his forehead. He lacked the stubble on his face, his trademark bandana, and often wore his school's uniform. Zenjūrō didn't appear to smoke, either.

Personality
Zenjūrō is somewhat of a pervert: he "especially likes the uniforms" of Japanese schools, and tells Kunieda outright that he thinks she's cute, and wonders if she has a boyfriend. When Hilda and Zenjūrō 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 Ittōsai Kunieda somewhat.

Plot
Zenjūrō Saotome makes his first appearance quite early on in the manga, in a flashback of Tōjō's in Chapter 37 of the manga. Tōjō tells Oga how he looked up to Zenjūrō, and ever since he first saw him when he was about 12 years old, he's always aspired to be as strong as him. Tōjō compares Zenjūrō 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 Zenjūrō. 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.

Zenjūrō Saotome 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 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.

Power
Zenjūrō 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 Tōjō, 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 Tōjō 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. He is so powerful that Behemoth also mention that there will be many casulties should their division clash with him. due to the amount of power he can use and the shape of the seal, it is reasonable to assume that he has formed a contract with a demon from the royal family. Due to their respective ages, he may be contracted with the Demon King himself.

Techniques
Spell Master techniques Shingetsu style:
 * Spell Bind: Demonstrated when Hilda attacked Saotome under suspicions he was with Behemoth's Pillars. He was forced to use this to restrain Hilda from attacking him further to convince he wasn't. The seal on his hand enlarges and pulsates, then launches what seems to be a duplicate of the seal towards Hilda which then wraps around her and then binds her. It can be assumed it diffuses a demon's powers as Hilda could not escape from it until it faded.
 * Spell Master Blast: First demonstrated when Pillars Naga, Hecadoth and Graphel appear. Somewhat similar to Oga's Zebel Blast, Saotome's seal enlarges and pulsates, then releases a giant pulse of energy. The true devastation of what the blast could've done was not shown as the Pillars had escaped via a Teleport artifact. However, with the fact the blast had wrecked half a street is enough to show it was powerful.
 * Nadeshiko - An armour breaking technique. Saotome could also do this technique, but with more power than Oga, being capable of cracking the ground and a big boulder.

Quotes

 * "Piece(s) of shit." - Saotome's usual insult/catch phrase.