Ao no Exorcist wasn’t going to be able to sustain my interest in the battle without resolving it for two more episodes, so I’m glad it was finished in one. And what a finish! All the little separate parts of the battle came together wonderfully, and Rin finally figures shit out.
As Shura says, the only think he lacks is confidence. He’s been holding back all this time because he was scared of who he truly was: the spawn of Satan. As the Observing-from-on-high Mephisto quotes: sometimes to defeat a monster you have to become one.
Dealing with Todou certainly requires vicious and vigilant attention, as Kurara enables him to regenerate right quick; if it wasn’t for the heavy rain (and added effectiveness of naiad bullets) Yukio & Co. would have been in bad shape. Again, though, this fight seems a bit of a distraction beside the Impure King threat, and Yukio’s realization that he shouldn’t be listening to the temptations of a demon comes curiously late in his battle.
Sick of watching Rin ineffectually hack at the seemingly infinite tendrils of the King, Ucchusma, noticing the Koma Sword, agrees to lend his power to Rin, shrinking in size, turning blue, and perching on the tip of the sword as a result.
But while Ucchusma wants the two of them to completely obliterate everything on the mountain—including all those who have been “contaminated”: Rin’s friends and comrades—Rin has something less destructive in mind; something only he can do: use his flames to defeat the Impure King without harming anyone or anything else. When he does so, the flames threaten to overwhelm him, tearing away his humanity.
Rin eventually calms down and remembers what happened to Shiemi when his flames enveloped her in Mephisto’s prison: nothing. So after Shiemi herself, after losing Nee again, climbs the rot that has enveloped Izumo, ignoring her insults, and pulls her free, and after Rin’s blue flames pour out in all directions and envelop them both, Shiemi is not afraid; everything will be fine.
Bon, Shura, Renzo and Koneko; everyone is bathed in the blue flames. At first they feared it would mean their death, but the flames are harmless, and only destroy the King and his Impurity. High above the mountain, Mephisto applauds the “bonfire” Rin created, happy the powers of the Blue Exorcist have finally awakened.
Shura, Renzo, Koneko, and the Myoda monks are all smiles in the aftermath of Rin’s great achievement, but there’s one guy who rains on the parade with a brutal punch to Rin’s face is Yukio, who is furious that he’s out of his cell. Obviously, he’s not up to speed, and Rin’s puckish grinning and laughing doesn’t help matters.
So Rin gets serious, and shows his little brother who he truly is; the thing he’s no longer afraid of facing himself. Shino never directly told Rin what he should do with his life, but left it up to him. So he chose: with the power he always had, but could not control because he feared it, he will keep his friends and family safe from the forces of evil, using Satan’s own flames against them.