Behemoth

is the leader of the powerful army that works directly under the Great Demon Lord and Lord En. He has been mentioned several times, but finally did his first appearance after the Oga/Beel Body Switch incident, looking for Oga in the Saint Ishiyama building.

Appearance
He has the appearance of a simple old man, though this could be only a disguise, as his powers were clearly threatening

Personality
Being introduced in his old man form he is shown to be calm to the point of not reacting to four opponents attacking him only to transfer to right in front of Oga to issue a war declaration. He claims that his reason for stepping down as general is due to old age.

Even though his son was removed from the Pilar Division due to his temper, Behemoth still reinstates him as the general because he wishes to see his son succeed him.

Akumano Academy Arc
Behemoth arrived to the Human world at an unknown time seeking Oga out at St. Ishiyama High. He met both Hiromichi Gō and Ichirō Shinjō and questioned them about Oga, after that, he had beaten them down with relative ease. He then proceeds to search for Oga. He was later ambushed by Ittōsai Kunieda, Genma Isurugi and Zenjūrō Saotome, though he didn't fight them back, saying that he only came to greet his new neighbors. He then revealed to have build the Akumano Academy in the grounds that used to be Ishiyama High.

Despite his earlier claims, once he met Oga face to face, he declared war on him and his group, but denied that he was the general of the army. He then introduces Jabberwock, who suddenly appears next to them, as the acting general. As his son summons Sodom he jumps on the dragon as well and discuss Oga's skills with his son. When Oga appears on Sodom he stands back and allows Jabberwock to fight him alone.

He later appears in his office at the Akumano Academy revealing to Hilda that she was captured because En wanted a new Wet-Nurse.

Mobichi Arc
Behemoth is summoned by Takayuki Furuichi during his fight against Tatsumi Oga. Upon his arrival, Behemoth hears an outspoken speech from Furuichi and decides to lend his power because of it. His immense power begins to physically affect Furuichi's body but he continues to fight, nonetheless; while his host is fighting, Behemoth stays out from the actual battling ground. At the end of the fight, Furuichi passes out and the summoning ritual has ended.

However, Behemoth's spirit remains amongst the two teenagers and the other Demons nearside them. While later toying with his mustache, Behemoth notices that Furuichi has awakened. He is then inquired as to why he still present by Hilda whom Behemoth assures that he will not do anything, especially considering how his physical body is elsewhere. As the conversation continues further, he explains that he held back from killing Oga as it would have resulted in Furuichi's body mutilating itself. Behemoth also mentions that he is surprised at how his contract with the Great Demon Lord turned out. Afterwards, Behemoth takes his leave. Before he disappears, he wonders in awe at Furuichi's abnormal endurance despite being a normal Human, but he shrugs off the thought before he thinks more about it.

Powers & Abilities
Although he hasn't been seen fighting, it can be noted that he is exceptionally strong, seeing as he was the leader of Behemoth's 34 Pillar Division. He was also able to beat back both Hiromichi Gō and Ichirō Shinjō by tying his shoe laces together and spinning them together to pummel the two. He is later on able to fight off Oga, Tojou, Kunieda, Shizuka and Jinno with ease whilst blocking and countering their simultaneous, continuous attacks.

Also, when Behemoth lent Furichi a fraction of his power it allowed him to easily overhwhelm Oga in his normal state then fight on par with Oga whilst he was using Super Milk Time. In addition, the small power Behemoth was giving Furichi caused his body to start breaking apart from the inside.

Trivia

 * Behemoth, in Jewish and Christian mythology, is the name of an unknown creature that is described as a "primal unconquerable monster of the land", controllable only by God.