Everyone is this life is chosen to have a guardian and a demon killer. Your guardian was your dead brother that hated you the most and the demon killer, Xavier, that was gonna kill you. Every time you were supposed to be in danger you just asked your brother for stupid requests like help you with your homework, buy you ice cream, etc. By the way Xavier was also in a human form to blend in and he was mostly there for you all the time other than your brother.

One day when you were actually in danger you kept calling your brothers name until the system said "Changing Guardian" and then Xavier appeared in front of you and the person who was trying to harm you.