Board Thread:Questions and Answers/@comment-26091443-20160702065117/@comment-28878284-20160704092809

I guess Gowther has been a Doll until the moment when he was somehow betrayed - which led him to join the demon clan and the ten commandmends and becoming a demon. (as Gloxonia and Dolor)

At some point he must have lost his emotions, maybe someone took them, which then led him to leave the ten commandmends and the demon clan. Since demons are a race with high emotional characters (though they're negative), Gowther was not able to stay a demon without them. That's when he became a Doll once more.

I've got the theory that every race can join the demon clan and become a demon somewhen in their lives, if their negative emotions get too strong upon themselves. Which means if you are fulfilled with hatred and betrayal and your whole life fells apart, you will be full of madness and anger and become a demon after some time :) That would explain how Gloxonia and Dolor could've become demons as well.

EDIT: ah I just thought of the fact that Meliodas can change between his demon-side and his human-side. It also relates on his emotional balance, as he has green eyes and is "human" when he's joyful and happy and gets his demon mark in moments where he suffers under a lot of negative emotions :)

tell me what you think about that :D