For only appearing in one episode of the series, Ariel had quite the character arc. First, he was a generous angel who started the halo-bearing tradition. Then it is revealed that he is actually a thief, and almost certainly a demon, on the run. In a key flashback, we see Adriel enter through a portal (presumably from hell), and close the portal behind him with the halo. The halo is difficult to track when inside living tissue, so Adriel hid the halo inside Areala to both hide it from his hunters and earn the protection and praise of Areala's comrades.
It is clear that Adriel is not a human, as he has survived in his stone prison for over a millennium. But the jury is still out on what type of being he is, and what powers he possesses. So the main question here is, who is Adriel and who did he steal the Halo from while in hell? There is a theory that Adriel actually stole the halo from Lucifer, which explains why he was being hunted so mercilessly. This would explain the halo coming from hell, but isn't the most satisfying answer.