Okay, I'm going to write this because I can't get it out of my head. What if Fai himself wasn't human? That would kinda of explain why he doesn't die easily. What if he is kinda like Chii, but Ashura was the one who created him. Maybe Ashura wanted him to do some kind of job Fai didn't want to do and that's why he sealed Ashura. Maybe he wished made a wish to Yuuko to seal Ashura. Just a theory...
Someone in a topic that I don't remember which one was, said that Fai could be older than one could expect, and when I was reading vol.3 of the Del Rey adaptation chapter 19, the theory grew more in me; when Kishim (that's her name, no?) tells them that they are children Fai says 'What a nice compliment!'. Maybe he is much older.