Two Equals One

When Satan comes to take Rin home to Gehenna, Rin has to think of some way to save himself and Shiro all on his own. Out of impulse, he makes a deal with Satan-after hearing Satan complaining about how he can't exist in Assiah because he destroys everything he touches, Rin suggests having a small piece of himself in Rin all the time-specifically, his head. Satan agrees as long as Rin doesn't give them away and no one is any the wiser as he starts as a student and Exorcist-in-Training at True Cross Academy.