Blue Exorcist

Rin and Izumo are attacked by a crow-like demon known as a gufu, but Yukio arrives and shoots it away. Mephisto orders the Exwires to check the thousands of barriers protecting the school looking for any breach. Rin encounters Konekomaru, who is shown to be possessed by the gufu, who took advantage of Konekomaru's fears to infiltrate the academy in order to convince him to kill Rin. The other classmates prevent Rin from harming Konekomaru, not knowing of the gufu's existence. Meanwhile, Shura locates a hidden door to a snowy forest leading to a suspicious abandoned laboratory. While in the hospital, Ryuji witnesses Konekomaru's heart succumbing to the gufu, who comes after Rin. When his friends rush to help him, Rin manages to slay the monster and save Konekomaru's life. Ashamed of causing so much trouble, Konekomaru decides to leave the academy and return home, but Rin reaches him before he departs and convinces him to stay, which helps him reconcile with the others. (Source: Wikipedia)