Demons are creatures created of a type of energy by the name of Malum. Demons have a natural perception filter that has lead to them going largely unnoticed to modern society. Through purposefully causing accidents or illness demons will eat humans on the verge of death.
Mari, our protaganist is a half demon and can see demons normally despite the perception filter. Though others with extensive knowledge of demons and their existance can see demons aswell. These people refer to themselves as 'exorcista'. But all exorcista would agree that it isn't necessarily good to know of demons as the dangers of knowing are much greater than ignorance.