Ao No Exorcist (Anime cut short) blue exorcistYuki found out that rin was strange and vowed to grow stronger to be able to protect him instead of being protected. Satan wanted to possess their father, fujimoto. His will kept him away from them all. Rin was always getting into fights and hurt a group of idiots who were shooting the poor pigeons. Finally rin went to teach at the exorcist school and left rin alone to find a job and stop getting into fights.
He got a job but he could suddenly see the paranormal. The people he beat up forced him to release his power and tormented him about yuki. They called him a prince and almost took him to the demons world. Suddenly father fujimoto came to rin's aid and helped him by chanting a prayer the lord is my strength and my shield, my prayer has been answered. Fujimoto explains everything to rin gave him a phone with his friend's number. He claimed his friend would protect him and he should pack his bags after giving him the demon slaying sword saying you must never remove it from it's sheath or release his full demonic powers as the son of Satan born of a human woman.
rin ran away and screamed at fujimoto saying your no father of mine and that hurt fujimoto dearly and broke his will. Satan possessed him and greeted rin and tried to take him to the demon world and called his power almost nothing. Fujimoto killed himself to stop satan from possessing him and taking rin. Caught in the demons gate rin has no choice but to release his demonic power. He pulls open his sword and broke the gate.
rin left the church and called fujimoto's friend. he was closer than expected and knew more about rin and yuki than he thought. He wanted rin to die until rin said he would defeat Satan and become an exorcist.
In the end yuki protected rin after telling him to die. They made friends but time passed to quickly.
Satan possessed yuki instead of rin and so rin fought to free yuki then to kill satan together. They succeeded their battle and losses not in vain as all the while the other exorcist were protecting the regular students and helping them get through it