The demons portrayed in the Blue Exorcist anime and manga range from lower-ranked to higher-ranked demons and Demon Kings. Aside from Satan, the Demon Kings are the most powerful demons in existence. However, as they inhabit vessels to manifest in Assiah, they can’t fully express their powers most of the time.
Even with the limitations of their vessels, the abilities of Satan and his Demon Kings make them some of the most feared characters in the series. Among demons, very few can threaten their standing. Whether it be Mephisto’s space-time manipulation ability or Lucifer's spears of light, the most powerful demons of the Blue Exorcist series are feared by many, and for good reason.

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8 Amaimon
The King Of Earth Cannot Access All His Abilities
Despite being one of the weaker Demon Kings, the King of Earth, Amaimon, is still much stronger than many high-class demons. In actuality, Amaimon is much stronger than he is portrayed. However, as he once destroyed the lands and made them inhospitable, Shemihaza, a high-ranking member of the True Cross Order, had to sacrifice herself to seal him, leaving only his sense of self active.
With his sealing, most of Amaimon’s abilities are sealed, and he can only access the bare minimum of his abilities. As a demon, Amaimon is often seen to be calm; however, he also has a brash and carefree side that pops up when he has his mind set on something.
7 Astaroth
Quick To Employ Violence
Of the eight Demon Kings, Astaroth is one of the more violent and sadistic ones. He embodies his nature as a demon and finds joy in beating and threatening people. Astaroth backs up his violent tendencies with his powerful abilities of rot.
As the King of Rot, Astaroth is capable of unleashing rotting curses and controlling demons associated with rot, such as ghouls and demonic mushrooms. Even without a human vessel, Astaroth’s abilities are potent enough to kill exorcists, even with armor on. Even with his sadistic tendencies, Astaroth is very loyal to Satan. His behavior towards Rin takes a complete turn after he finds out he has Satan’s blue flames.
6 Egyn
Quite Childlike, Even As A Demon King
Unlike the other Demon Kings, despite his immense powers, Egyn has many childlike characteristics. He has a small frame, likes to hide his face, and is uncomfortable around strangers. Of all the Demon Kings, Egyn looks the most similar to Iblis, and the two of them get along well.
As the Assistant Director of the Illuminati’s research lab, Egyn plays a secondary role in Blue Exorcist. As the King of Water, Egyn’s abilities are water-based. He can appear out of water puddles and form barriers out of water. As a Demon King, Egyn is powerful; however, he hasn’t been seen using his powers for offensive purposes.
5 Iblis
The King Of Fire Can Easily Dispatch Most Arc Knights
The King of Fire, Iblis, has made few appearances in Blue Exorcist. But even with her few appearances, she has proved herself to be exceptionally powerful. Interestingly, Iblis is the only one with a female vessel among the eight Demon Kings.
Unlike Mephisto and Azazel, Iblis is an ally of Lucifer and is against the True Cross Order. While Iblis does consider Rin Okumura to be cute, she generally dislikes Nephilim and would rather not interact with them. In terms of ability, Iblis' powers deal mainly with fire, and she easily defeated Osceola Redarm, an Arc Knight and exorcist of the True Cross Order.
4 Azazel
Even In A Form Of Cryo Sleep, Azazel Is Still Highly Regarded
While little is known about Azazel, he was once known as the third strongest of the Demon Kings. However, some claim that he has returned to nature, as he takes on the form of an elderly man with hair that has obscured most of his features. Azazel is also in a crystallized form, which makes him immobile. As such, he only takes on support roles and does not directly combat anyone.
Being the King of Spirits, Azazel is known to interact with ghosts and byakko, low to high-ranked demons. He can communicate with various kinds of spirits, and while the extent of his abilities is unknown, the respect he has garnered speaks volumes.
3 Mephisto
Control Over Space & Time Makes Him Nearly Unbeatable
The King of Time is known by many names, such as Mephisto Pheles, Samael, and even Johann Faust V, which most humans know. As a demon and exorcist, Mephisto is quite an intriguing character in Blue Exorcist. While his true motives are unknown, he broke up with his father and siblings and now serves as the principal of the True Cross Academy, an academy for training exorcists.