Board Thread:General Discussion/@comment-6192846-20140918010005/@comment-6192846-20141225045304

The only demon we've basically gotten a confirmed "true form" for is the original Demon of Gluttony and perhaps Banica. I don't think any of the others have ever been pushed so far to their limits that they've revealed their true forms behind the abstract blackness that usually clouds their appearance.

All the gods seem to have a human form (at least in the second period) save for Sickle, who also is the one incarnating the other three into their third period forms (though Levia-Behemo might count as having a third form with their formless existence inside Elluka).

We know very little about demons, specifically Demons of Sin, in this series so who knows.