User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-7418318-20160926124441/@comment-84.197.239.101-20160926204338

He has seen Lucy naked or at least bare-chested before, but maybe his "demonic side" isn't used to it yet. Seems like Zeref inserted a Jacob personality into E.N.D.

If that's the case, then the key to breaking the "demonification" is fanservice, I mean Love.

All they have to do now is let Natsu deal with Zeref and quickly have Lucy strip in front of him once Zeref is dealt the finishing blow and Natsu will be freed from the Curse right before he fades out of existence.

Laughs and jokes aside, that would be the first time the fanservice has a purpose plot-wise.