Power User is the term used to design those individuals with abilities that can be clasfied as supernatural. There's no need for a Power User to be human, as they can range from animals to robots, though humans are the most common.
Power Users are scattered all around the world, Helise having almost all of it's population made of Power Users.
There's no physical explanation for powers, but it's known that they are powered by the mind, most especifically the User's will, meaning that fights among Power Users will ultimately be decided by each User's willpower. Their powers seem to be completely random, having no relation to the User's traits like personality or appearance, they simply happen.