Power Users is the term given to those with special abilities in the manga.
Power users can be traced back to the Meji Era, when the government learned of their abilities and started to gather them in order to experiment and bring under their control as weapons. However during the progress of imperiallism, the soldiers oppressed them and in secret, began to "purge" several power users who did not follow their instructions, viewing their abilities as threats to humanity.
Generally they are feared for their abilities, mostly because people cannot understand why they have such powers. As such, most users are often oppressed or tortured in many ways, as Lily has shown, "a way of purging their uncleanliness. Thus quite a number ended up resenting the normal humans, and began to kill/torture them. Some even gave the justification that having the powers means they are the true rulers of the world, and that they have the right to cleanse the world.
Most, if not all power users, utilise abilities which are mostly of either energy or conditions that exist in the world, such as sound and air. There are however exceptions to this rule, such as the seven flames and the rare kind.