I think that everyone should be open-minded as to "what/who Yuya is" because as atkins513 said, there is no definate answer. Hyu will simply have to find this out for himself and trust his beliefs as to why she is in his dreams. Anyone who says "no, that's not right because science says blahblahblah" is rather close-minded in my opinion.

Do you know everything there is to know about dreams and this world?