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.

In a fit of rage, five year old Okumura Rin unlocked his secondary powers, and felt as if he was forced to flee because of it. While sitting in a cemetery tree, a man who claims he's Astaroth tells Rin his real father wants to meet him. Feeling helpless, Rin agreed to go with the man, and so his life in Gehenna begins.

Rin overhears a conversation of his supossed family. They talked about him and his powers growing. They never noticed Rin standing near the door way inside the room. It wasn't new for him. They barely paid attention to him. Which is on of the reason they didn't know that Rin isn't dumb, but actually smart. What will happen if a sceret kept from him get's revealed? What if everyone of his family exept for one turns against him? Got the idea from a TikTok :)