Episode 18

Gufu (颶風) is the 18th episode of the Ao no Exorcist anime overall and of the first season. It first aired on August 14, 2011.

Synopsis
Rin and Izumo are attacked by an unknown enemy and Mephisto Pheles orders the Exwires to check the barriers protecting the school looking for any breach. The enemy is in fact a demon who managed to take advantage of Konekomaru's fear of Rin to infiltrate the Academy. Meanwhile, Shura locates a hidden door leading to a suspicious abandoned lab. When Konekomaru's heart is finally taken by the Demon, his friends rush to help him, and Rin manages to slay the monster and save his life. Ashamed of causing so much trouble, Konekomaru decides to leave the Academy and return home, but Rin and the rest of his friends manage to convince him to stay.

Trivia

 * Rin is more sensitive to the presence of demons than normal people due to his demonic heritage. He could see the Gufu when others couldn't.
 * Konekomaru didn't tell anyone about the Gufu due to it manipulating his fear of Rin.

Manga & Anime Differences

 * This is an anime only episode, none of the events of this episode happen in the Manga, except Izumo's conversation with Rin about his demon heritage. It is taken from Chapter 17, with the difference that they were aboard a train, along with their classmates.