From the introduction:
Since 1997, therianthropes began appearing around the world as people fell into despair; mutating monsters who cared mostly about feeding and destroying. These former humans had given in to despair, hatred and anger, transforming into barely-humanoid monsters. At roughly the same time, girls began transforming into Heart Warriors to fight off these creatures.
The main threat to the world remains the seven most powerful therianthropes: the Chayot Hakodesh, the city-killers, against whom even the strongest Heart Warriors seldom stand a chance.