User blog comment:Miskos3/Fairy Tail 535: The Greatest Power, Review/@comment-5303512-20170514221549/@comment-4933386-20170518074200

Demon = the person. Not a separate thing that takes possession. It's literally same as all the demons we've seen. It's 100%, no one is possessing them. It's just Natsu suddenly got the demon seed growing inside him, that's triggering his demonic abilities.