Fai ending up with Lischen would be the most interesting option so far. She would constantly be under a huge temptation to read his mind and memories to learn what on Earth was the glimpse of chaos that made her throw up, and Fai would conversely try to stop her from doing it, for her own sake, yet at the same time maybe he would actually find it comforting if someone else knew that other world as well. Knowing it would be the only way to really get close to him because it defines his personality.