Humans- Self thinking beings that lack souls, though are self aware and free thinking

When a demon and a human mate the child will have a demon soul, which desires a certain form. As the child grows the soul and the body will fight each other, resulting in health issues and possibly mental issues as well (basically it is a square peg rammed into a round hole).

If a soul is unsealed by a ceremony, then the body will morph to conform to the soul and the person will become a demon (making the round hole the perfect size for the square peg). The transformation is extremely painful and the person essentially dies (the soul does not need organs to function) so a demon will have no pulse and not need to breathe either.

If a soul is never unsealed it will end up destroying the body and never be able to transform, and the soul will disintegrate. This is also very painful and the people with this fate often go insane from their body being ripped apart from the inside (the peg and the hole end up being destroyed)