User blog comment:CNBA3/Fairy Tail 411 Review/@comment-7418318-20141201122317/@comment-9595671-20141201154011

I kinda see one thing that makes the demon theory less likely, Mard's reaction to it. That and what Zeref said to Natsu earlier on in the arc.

I mean if Natsu was a Demon then it would make sense that Mard would have realized it, the demons are naturally stronger than humans so he'd have noticed this. This is also why I don't think the END possessing Natsu theory works other then I've not seen a single demon possession in the series.