( I DO NOT OWN AO NO EXORCIST / BLUE EXORCIST ! )
It all started soon after Rin went to True Cross Accademy.
He had a normal life untill he found out he was a demon.
He attends True Cross Accademy with his twin brother, Yukio.
A month after he joinend, you came in. You came wanting to become an exorcist because of your past. You were like all the others, but there's something specials about you that Rin liked.
He didn't know what, but he just did. You were a demon just like him, and no one knew except Mephisto and some of your Teachers.