Genetic Prototypes.
Created by the Emperor to serve him, these were the strongest of humans. However, in reality, the Genetic Prototypes, in the role of the Four Heavenly Kings, were not entirely loyal to the Emperor. This is quite understandable, and many players also think that it was a mistake for the Emperor to create the Genetic Prototypes in the first place.
According to historical records, when the Emperor first unified Terra and created the Empire, neither the Star Warriors nor the Genetic Prototypes had appeared; at that time, the Emperor's subordinates were a group of super genetically enhanced warriors known as the Thunder Warriors and the Imperial Army Corps. Only after the Emperor unified Terra did he begin to create Genetic Prototypes.