Blue Exorcist Season 1 Episode 18

Rin and Kamiki 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 revealed to be a demon who took advantage of Konekomaru's fears to infiltrate the Academy. Meanwhile, Shura locates a hidden door leading to a suspicious abandoned lab. When Konekomaru's heart succumbs to 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 reach him before he departs and convince him to stay.