User blog comment:IamJakuhoRaikoben/Chapter 373: Life or Death?, Review./@comment-5303512-20140301213856

Regarding the theories of Natsu being E.N.D, I only have one problem, that would mean he would be commanding the Demons every move, and Natsu would have to be away from Fairy Tail for a while. Probably at the time between The Dragon Invasion ended and Natsu appeared dressed as the king.

My Theory (2nd likely possibility) is that E.N.D is inhabiting Lisanna, because I find is surprising that she is getting more screen time since Edolas or Tenrou. During the time she was transported by the Adema, E.N.D was in need of a host and the injured Lisanna fit the bill.

My main theory is that E.N.D lost his body and goes about taking human hosts until he can find his original body. Current host someone relevant to Natsu.