Here is a challenge on Ao no exorcist for those interested.
The idea is this: Rin holds a secret that all ignore. Indeed, Mephisto sealed the demonic powers of Rin, but all have forgotten that Satan being an angel before becoming a demon. However, only Rin is aware of his powers (which he perfectly master) and decides to keep them hidden as a child, for he believed that this was proof that he was a demon. Finally, he is supported in this belief when he learns that his father is Satan and decides to keep the secret since he does not want to be seen by others as being more dangerous than he already is.
Here's what the fanfiction should contain:
1) History must remain as close as possible to the manga
2) The couple is a choice, but I encourage Mephisto/Rin stories
3) Shiro does not die from the possession of Satan, he is only plunge into a temporary coma
3) The secret of Rin can come out in two different places in history either during the fight between Rin and Amaimon at the training camp or during the fight against the impure king
Good luck to everyone
