User blog comment:FairyRaven/Fairy Tail Chapter 414: Drops of Fire, Review and Commentary/@comment-1776824-20150105154224/@comment-7418318-20150105155249

For me, I'm now more convinced than ever that Natsu can't be E.N.D.

Furthermore Zeref said two things here. That he wanted to retrieve END (he said END, not the Book of END!) and that he wanted to finish his fight with Natsu. To me it seems that Zeref is making a very clear distinction between Natsu and END here.

Then there's also the book and body revelation we got from Mard. As long as Mard's body was there, his book wasn't. And the moment his book appeared, his body was gone. Thus I think that a Demon doesn't actually come in and out of the book, but that the book is turned into the Demon and back, just like the flute became Lullaby and the Demon turned back into a flute after being defeated.

But we've clearly seen Natsu and the Book of END exist at the same time, thus I don't think that Natsu can be END.