Mephisto Pheles (メフィスト・フェレス Mefisuto Feresu), whose real name is Samael (サマエル Samaeru), is an Exorcist and the second strongest of the Eight Demon Kings; he holds the title The King of Time (時の王 Toki no Ō). He uses the name Johann Faust V (ヨハン·ファウスト五世 Yohan Fausuto Gosei) in public and is the Principal of True Cross Academy. Following the death of Shiro Fujimoto, Mephisto takes it upon himself to enroll Rin in True Cross Academy.
Mephisto has dark purple hair and forest green eyes. He is also very tall, having a height of 195 cm (6'5"). He is usually seen in an elegant white outfit that consists of a white top hat with pink and purple around the base, purple gloves, a tie that is pink with white dots, a white suit-shirt and undershirt, jester-style pants, and pointed shoes. His appearance prompts Rin to refer to him as The Clown. As the series progresses, he stops wearing the jester pants and pointed shoes, instead wearing white dress pants and black shoes.
In addition, he can transform into a small white dog when he counts down from three in German (up to three in the anime). In this form he is a small, fluffy, Scottish terrier-looking dog with a large pink ribbon around his neck with a silver trinket attached to it, the trinket being the symbol of an Exorcist.
Shrouded in mystery, one quickly learns of Mephisto's innate ability to act as if he knows more than what he is letting on. Careful and meticulous, he is like a chess player, always ahead of the game.
He enjoys chaos, as he stated to himself that he was a hell-raiser, and Assiah to be his playground. He frequently observes the growing conflict around Rin and his friends with delight, though he does not wish to see any of his students get killed. He apparently 'loves' humans. When Mephisto first meets Amaimon in the series, it is implied he is conspiring with Satan and is most likely training Rin on his orders. He likes to make gambles with whoever possible, including Fujimoto, Rin, and the True Cross Order.
He is prone to eccentric moments nestled in between serious moments, making light of a heavy mood and is portrayed as a man with more money than sense, but is very cheap when it comes to spending on others. Like his brother, he loves to eat sweets and he is deeply interested in the Japanese pop culture, proudly stating that he is an otaku (that is, loves computers, anime/manga, and/or pop culture to the point that it negatively effects his social life).
According to his own words, Mephisto has wandered Assiah for an unknown amount of time. He has apparently been known as Loki, Trickster and Raven at some point during that period of time. Around 200 years earlier he became known as Mephisto Pheles. Other than the fact that he was close friends with Shiro, and that he has assisted the Vatican for around 200 years nothing much is known about him.
The next day Mephisto comes for Rin in a pink limo, almost running him over, and he takes Rin and Yukio to True Cross Academy. On the way there he reveals that Rin is going to attend school, as well as attending the cram school where he will study to become an Exorcist. When Rin finishes his tour of the academy, he sees a dog who makes Rin follow him. After they get out of the academy, the dog transforms into Mephisto and explains that the principal cannot randomly walk around the school. He told Rin that he would accept him in True Cross Academy under the one condition that he keep his identity as the son of Satan a secret.
Mephisto then gives Rin a magic key, that allows him to use any door get to his class, and transforms back into a dog to accompany Rin on his first day of class. Yukio enters the classroom as their teacher and Mephisto explains to Rin what a Mashō,a wound inflicted by a demon, is and he also explains that it is required to receive a Mashō in order to become an Exorcist. Mephisto then stays in the classroom and watched as the two brothers start arguing, and he continues to watch as Rin accidently makes Yukio drop a vial of fermented animal blood that draws a horde of Goblins to the classroom. Much to his amazement, Rin and Yukio work together to slay the Demons after the other students had been moved out of the room. He later tells Yukio he was a bit "stiff" for teaching his first class. He then mutters to himself that Satan would make a move soon.
Later on, he is seen having a conversation with his brother Amaimon. They talk about Rin and the rest of their family in Gehenna. Satan has accepted Mephisto's proposal that he will take care of their youngest brother, Rin. Amaimon then asks when he will return to Gehenna and Mephisto replies that he has a lot of work here in Assiah. After Amaimon leaves, he says that this world is his biggest play toy.
Mephisto appears half way through the arc floating above the forest with Amaimon, using his transformed umbrella to do so. Amaimon is there to yet again test Rin and talks about how he wants to kill the people below. Mephisto warns him that if he kills any of them he will kill Amaimon, making his brother restrain himself. He watches the battle from above until Rin and Amaimon get out of control and complains about what troublesome little brothers he has; they were causing property damage to his school, which was something Mephisto would not tolerate. He descends to the battle and summons a cookoo clock that he uses to imprison Amaimon. He vanishes the clock and then subdues Rin by sheathing the Kurikara. At this point the new Paladin, Arthur A. Angel, arrives and arrests Mephisto and Rin to bring them before the Grogori. Mephisto takes this lightly, changing into a respectible outfit before going with Angel to be tried.
Mephisto is brought before the Grigori and accused of betraying the Vatican by lying about the existence of Satan's sons, mainly Rin. As Rin is kept subdued by Arthur A. Angel Mephisto gives a speech about how it was done for good and that Rin may become the weapon the True Cross Order needs to switch to the offensive in the war against Satan. He asks the Grigori to take this bet with him. They agree to an extent, Rin must pass the Exorcist exam in 6 months or be executed.
Mephisto appears briefly in the start of the arc to tell the Exwires they are going on a real mission. He does not reappear till after the Demon's revival. He shows up to capture Rin as per the orders of the Vatican, who have decided that Rin should be executed after having lost control of his flames again. He provides Shura with cloaks that turn the wearer invisible, implying the students are to save Rin. Rin is rescued from Mephisto's prison by his friends and heads off to battle the Impure King himself and prove to the higher-ups that he should be allowed to live.
Mephisto, apparently having planned this turn of events from the start, prepares to watch the ensuing battle from afar. When Rin kills off the Impure King, Mephisto laughs, stating that this has brought upon Rin's birth as the Blue Exorcist fated to kill his own kind. He later is seen talking with Yaozo and Shura, apologizing for not being there to help, and if Yaozo is called by the Vatican, to not hesitate to telling the truth. When he leaves, Shura attacks Mephisto, holding a blade to his neck, and demanding to know his plans. He then tells Shura to value her life, as it would be a shame if a beautiful woman died so young. Realizing what he is implying, she then leaves.
After Rin arrives for a dinner at his Mansion, Mephisto reveals that he have been called many things over the years, but his true identity is that of the Demon King of Time, Samael, who is second in command of Gehenna.
Mephisto is seen talking to a injured Shura.He says he busy fixing the barriers and tells Shura the Illuminati abducted Izumo and the spy Shima.
Mephisto summons Rin and Yukio to his office to investigate the whereabouts of Shura Kirigakure.
Blue Night Investigation arc
At the start of the arc Mephisto meets with Shura. He tells her that he is glad to see her return and is surprised to see that her sword has maintained it's form even after Hachirotaro Okami was defeated. Shrua then tells him that it's just an empty shell and that she should be demoted. Mephisto then offers to have a high class demon possess it. She tells him not yet and that she will need a scabbard for it and to please handle her demotion. Mephisto acknowledges her request. As she is about ot leave she asks if they found Hachiro. Mephisto explains that the Illuminati seems to have got him first.
Surprised, Shura then questions him and he explains that Rin's flames had weakened Hachiro enough that he was able to slip out of the seal and that a search party was formed but they spotted an Illuminati helicopter leaving the area and due to stealth technology, they were unable to track it. Shura asks is Renzo was behind it and Mephisto tells her that he does not know. Shura then accuses Mephisto that this is all part of some plan of his. He tells her he noted that her demeanor has changed and she throws her sword at him and just barely misses. She then threatens him and he tells here that she has indeed changed. She say that she is even surprised by her own feelings but that a demon would not understand.
Later in his office, Mephisto summons a doorway and opens it. He then asks whomever is on the other side if they have cooled their head yet. Much later, Lewin barges into Mephistos mansion and forces his way into the bathroom where he then begins to question Mephisto about Section 13 and the meaning behind those that have Molinas contracts placed on them without any records of such. 
Meister & Abilities
Mephisto is powerful enough to block any higher ranking Demon from entering into the academy. It is stated when Pheles told Amaimon the King of Earth, to enter into the academy, Amaimon replied that he can't enter into the academy without the "Infinity Key" because of Pheles' barrier. How powerful he is has not been shown yet but he has mentioned that he is an Honorary Knight, the position two ranks below Paladin. When he sealed Rin's blue flames into Kōmaken, he only had to snap his fingers.
Transformation: Mephisto possesses the unique and peculiar ability of transforming into a small dog after counting down from three in German. This form allows him to escape through small gaps and spaces. He has also shown the ability to transform into a cockroach, which he uses to spy on a conversation between Yukio and Shura.
Time-Space Manipulation: As the King of Time, Mephisto has dominion over time and space, manipulating it in various ways. He is able to transfer his and others' consciousness into other dimensions, such as Gehenna. Also, he has shown to be able to stop time for an unknown period of time allowing him to snatch Rin's sword.
- Er Stopfen (
時よ止まれErusutofuven): A barrier that stops time from passing within it, it lasts about four to five months.
Summoning: Mephisto is able to summon a variety of objects by counting up to 3 in German.
- Kuchens Kuckucksuhr (
お菓子の鳩時計Kūhenzu Kukkusuwā; Ger: "Cake Cuckoo Clock"): A giant candy clock, it has a large cuckoo which springs out and grabs the target. It is first used to capture Amaimon.
- Das Stärkste Gefängnis (
一番防御力が高い牢屋Dasu Shutarukusute Gefengunisu; Ger: "The Strongest Prison"): A prison touted as the "strongest defense," it has the ability to halt the movements of those who approach it with hostile intent. It cannot be opened from the inside without the key but can be easily opened from the outside. It is first used to imprison Rin.
Flaming Pentagram: When the Boys' Dorm was attacked by the masked person, Mephisto said that he was going to make the person regret attacking the academy. He waves his umbrella in circles above him and a giant flaming pentacle appears in the air. It creates lightning as it hovers. It, like his other abilities, appears to be activated by him counting to three in German. However, Arthur Angel puts out the symbol with water, so it is currently unknown what it would have done were it to have been used. (Unnamed)
Telekinesis: Mephisto show the abillity to freely move objects he summons, such as making his chair levitate, flying a rice cup in the air, and more.
Appearances in Other Media
After subduing a rampaging Rin, who broke Amaimon out of Mephisto's prison, is imprisoned in a large crystal when Mephisto gives his speech and Amaimon attacks during the trial. Mephisto refuses to help until the Grigori agree to his terms on Rin. In the mean time Rin is revived by the repairs done to the Kurikara and defeats Amaimon, destroying his current body. As Amaimon is defeated he calls for his brother, Mephisto, to help but Mephisto stares on impassively.
Later on, while the Gehenna Gate is opened, Mephisto and Amaimon watch the events from Mephisto's floating chair. When Rin and Yukio create a giant blue fire bird, he seems delighted. After the Gehenna Gate is destroyed, Amaimon tells Mephisto that it's gone, but Mephisto seems distracted. Mephisto tells Amaimon that he remembers a man who he had made a gamble with a long time ago.
Mephisto won, but he remembers the man's dying words. The man said that he wished to live that moment forever, and Mephisto claims he now understands what he had meant. According to Rin's narration around the end of the episode, Mephisto went back to being the principal of the Academy, though still under his pseudonym, Johann Faust.
- Both his name and alias, Faust, are taken from German literature called Goethe's Faust in which the main character, Faust, wagers his soul with Satan.
- Furthermore, the Demon Mephistopheles in Goethe's Faust is able to transform into a small dog too.
- He also shares his birthday with Johann Wolfgang von Goethe
- Mephistopheles in Goethe's Faust is the manifestation of contradiction. In Ao no Exorcist, this is featured by him praising the value of high quality food in the school cafeteria, while he is cooking instant noodles, him wearing a unique white attire, while the students wear black uniforms and of course, him being a demon leading the True Cross Academy.
- His real name, Samael, is an archangel in Talmudic lore who fell from heaven, and is also called an angel of death.
- In Judaism lore, Samael is the guardian angel of the Biblical character Esau, brother of Jacob. In some theories, the angel that Jacob wrestles (from whom he earns the name Isreal) is named Samael.
- His average hours of sleep is 1 hour.
- He is pretty good at party games, claw crane games and chess.
- His pastimes and talents are: watching movies, reading manga, watching anime, playing video games, listening to music, toys, and subcultures.
- All manga genres are his favorites.
- His favorite food is junk food (mostly candy).
- His favorite type of women are those who are temptresses, "devilish", or elegant/beautiful. In the Weekend Hero novel, he mentions also fancying "pretty girls with a sad past", as well as charismatic women.
- He has a frequent tendency of breaking the fourth wall by directly speaking to the reader.
- He likes to make gambles with whoever possible, including Fujimoto, Rin, and the True Cross Order.
- (After his talk with Yukio Okumura) "Dear me.. you've got too much tension in those shoulders, Mr. Teacher... You should try to enjoy life a little. And it's not just the order we have to contend with... it won't be long before Satan makes his move."
- (After his talk with Amaimon, in reference to Assiah) "To a libertine like me... there could be no better playground than this! The real fun is just beginning."
- (About Rin Okumura) "It's the birth of a Blue Exorcist... whose self-imposed fate is to kill his own comrades."
- (To Shura Kirigakure) "How old are you again? Please treasure your life. It would be a pity if a beautiful woman like you died at such a young age."