User blog comment:Omega natsu2/Chapter 413: The Book of E.N.D., Predictions./@comment-69.146.147.19-20150423052124/@comment-7418318-20150423182059

We got this cryptic message in which Zeref refers to Natsu as "Etherious Natsu Dragneel", but that's the only thing we have.

To make matter more confusing we've got contradicting matters here. Thanks to Zeref booking-in and burning Mard Geer we've got confirmation that a Demon can only exist as either the book or the Demon. If he's in Demon form, there is no book and if he's in book-form there is no body. In other words when awakened the book transforms into the Demon body and if put to sleep the demon body transforms into a book, rather than one coming out of the other.

Despite this we have both Natsu and the book of E.N.D. existing at the same time, but we have no explanation for it, just a couple of different speculations.

One of the more popular theories is that Natsu is only part of the whole Demon and that's why it's possible for both a body and the book being there at the same time.

Another (or the same depending on your point of view) is that E.N.D. somehow got his soul and his body split apart. Natsu is the body and the soul is the book (or in the book).

Though there are plenty more theories, not least of all that Hiro Mashima trolled us and Natsu and E.N.D. are actually completely seperate beings.