User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-6904925-20150518110318/@comment-7418318-20150518200302

Yes they share blood, but is that really important?

There doesn't seem to be any of the original mind of Zeref's brother in Natsu... only an empty body filled with a new soul/mind/being.