Regulus Ginedrive Immoral Egersis is the official name of a group of species created and deemed failed byproducts by the Jaula Blanca Royal Scientific Research Institute. These beings are called Reggies (レジー, Reji), for short.
According to Koku, Reggies have their name derived from an ancient and powerful race of demi-humans who were created only for killing.
The Reggies are failed byproducts created by the Jaula Blanca Research Institute from a series of experiments that were conducted over hundreds of years to revive a set of ancient winged gods. Originally known as the 'Promised Children,' these groups of individuals were normally kept locked up. However, as funds began to dwindle due to the king refusing to finance the Institute anymore, the leader of the institute came up with the idea to use the Promised Children as weapons to balance out the chaos and order of the country, leading to the creation of Market Maker.
As to date, Laica/Real Minatsuki is the only "Perfect Reggie," who was stabilized via medical treatments enough to be the host to the Black Winged King's right eye. As a result, Laica became the main focus of the attack on the Jaula Blanca Royal Scientific Research Institute because the government feared a perfect product. Keith Flick surmises that this may have been what the government had originally intended as they may not have been interested in a successful product, which Gilbert later confirms.
All Reggies are born with the innate ability to use enhanced reflexes and skills such as super-strength and speed. They also conceal an extreme innate blood-lust which normally lays dormant until it fully awakens upon the Reggie's 20th birthday.
After a Reggie's 20th birthday, the Reggie will start to gradually lose their mind and become more violent and mentally unstable as a result. Although most Reggies are able to keep their murderous urges in check by using a specialized drug that regulates their testosterone and dopamine levels, the drug doesn't work forever and will gradually lose its effectiveness over time until the Reggie becomes something similar to a wild beast.
Once a Reggie has becomes completely useless, the government releases them out into the wild. The only way to kill a Reggie is by using weapons made out of blue metal.