Well I would say, Hidden are humans with abilities, they are no different from Altered or other humans in any way, at physical/biological level. So physiologically Estia, Cleos and Heramael are all having human bodies.
And again, none of them are on Earth, I am pretty sure, our planet was not born anything like collision of rocks, like told in the prologue. So none of them are really humans to start with...😅