Fallens. The lowest rank there is, because they aren't really Underworlders. They are Angels who have chosen to descend down to Hell. The amount of Fallens had a significent increase when Aaron took control of Heaven. Fallens are led by Fallen King, who is best described as "Giant TanQR".

Shinigami. The second lowest rank, Shinigami can control death. Shinigami are not immortal, But they can only be killed if they: 1) Kill someone to save another person or 2) Don't kill anyone. Because of this, Shinigami are commonly used in war, as an unbreakable defense.

Demons. Medium-Rank of the Underworld. This means you have a considerable amount of power. Demons are not to be underestimated, As some of them can evolve into a Devil.

Devils. High-Rank Underworlders with a lot of power. You SHOULD NOT underestimate a Devil. It will be your dying act.

Archdemon. Practically a tier of your own in terms of power. You will rise above every other Underworld being. The strongest ever Archdemon recorded is Sirius, also known as Primal Green.