I thought Shelly was made from a dead body with magic
Skelly*
In the past they said skelly has a normal form but his demon father take some mana on him that why he is skeleton.
Maybe he is half human half demon so inorder to make him safe in demon word his father Did it.
Always thought he was a Chimera since... that episode where we saw Mikael's flashback on how Skelly was "born" he just took him from those "raining pile of bodies"
Or maybe I'm just tripping
Hahahaha
Skelly is their offspring