Blue Exorcist

Shura Kirigakure, a high level Exorcist and former apprentice of Father Shiro was acting as one of Rin's classmates tasked to investigate his connection with Satan. With orders to kill him if it were proven true, she is divided between fulfill her obligations and follow Shiro's request to look after Rin in his place if something would happen with him. Her doubts are cleared when she confronts Rin and finds that Shiro was actually raising him like a son instead of a weapon.