User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24784846-20150518115847/@comment-24039250-20150518155303

In basic terms as to who END is to Zeref, it's his bio-magical son (demon baby made by his magic) and his biological (since the base body is that of his deceased brother) brother all at the same time.