Demon Lord

A Demon Lord refers to a demonic being who has earned their title through raw strength and status.

Originally from the underworld, demon lords are powerful beings than can be found on either side of the moral spectrum. Demon lords often work under the demon king, but 'rouge' demon lords are fugitives who, once caught, are given their punishment in hell. Demon lords are the central antagonists in the main story.

Origin
If a human is convicted to hell upon death, they are either punished for eternity or work their way up the demonic hierarchy. Once they've earned enough merit, they are granted access to the surface world to roam and interact with humans. Some demons manage to make unwarranted exits out of hell.

Demon lords are powerful enough to steal human souls, which are the key to immortality. As such, some demon lords go 'rouge' from the underworld law and indulge in mass murder. It's been said that while human souls taste just "fine", the souls of half-human, half-creatures are "tastier".

Goals
Many demon lords seek immortality, which requires the sacrifice of many human souls. It is not stated how many souls are required to achieve an 'undying' state, but lords like Cyril have killed by the thousands and has yet to receive it.

Others may seek resurrection, or the act of bringing someone back to life. Resurrection is highly forbidden within the natural and supernatural world, and allegedly requires more souls than immortality does.

Lords
These characters may vary in strengths, goals, and morality. However, it may be noted that they are all from hell and have had devil wings at some point in time.
 * Watanabe Emiko
 * Cyril
 * Rei's Mentor

Shells
Killing a human is not required to obtain their soul, but a demon lord may choose to kill them regardless. In the rare case where a lord takes a soul but allows the human to live will cause them to become a shell. A shell is oftentimes a human living without a soul, void of any emotion or ambition. They become insusceptible to pain and essentially become zombies, making them the ideal throwaway soldier. These individuals will often become criminals or people of red-light districts.

Demon lords are often known to use shells. Rei was told, by his demon lord mentor, to find and use shells for his own protection.
 * Jun
 * Yuka

Shades
Whereas shells are the remnants of a soulless human body, shades are the souls of grieving humans who have died with regrets. They are considered emotionally daunting to use as they constantly test the user's mental state and hinge on their thoughts. However, since they are without a physical form, they are known to be more powerful and efficient to use than shells. Demon lords who use shades will often inflict the shades' regrets on their target, making them think the regret was their own.