Assassin!
Assassin! takes place in a universe that is populated by humans and supernatural creatures alike. It is the duty of the government-funded organization PURGE to weed out the dangerous nonhumans from the harmless ones.
More information can be found in this series' World.
Residing in the underground city known as Gehenna, the Assassin! universe's rendition of Hell, almost none of these characters actually interact with the main storyline or provide anything too meaningful to the lore, so they exist in a sort of spinoff series revolving around their lives and antics down below. Their stories progress throughout the main Assassin! storyline and after its conclusion!