User blog comment:CNBA3/My Big Three Theories!/@comment-67.149.177.130-20150217175104/@comment-5303512-20150217183715

Though as a shell, something has to be able to drive the body the E.N.D and be able to give it it's own personality.

Another throught is maybe E.N.D is the only demon that can it was created differently from the other demons that would make it the "masterpiece", even Zeref said that it was precious to him. Maybe E.N.D is like a actual child of Zeref.