A Monster (モンスター, Monsutā) is a soul type in which the quality of a soul is "monstrous". Like that of the real world, beings classified as "Monsters" are also typically frightening in appearance though this is not always the case, such as Blair being considered a Monster despite not looking frightening.
Not much is relatively known about an individual with a monstrous soul. An individual with monstrous soul has both been shown to possess a human-like form with possibilities of transformation too into their respective theme dependent on their race, such as Blair possessing a human and cat form from being a Monster Cat while Mosquito is a Bloodsucker able to transform into a bat in his Form from 400 years ago. Although some Monsters share similarities with witches, including magical abilities, they are different entities.
Other Monsters, however, are portrayed to simply have a frightening appearance much like the real world's iteration of a monster with no special qualities.