The Guardians were an Earth Federation organization established to help seijins in Earth Federation territory learn to control their abilities for the betterment of themselves and Earth Federation citizens at large.
It is the position of the Guardians that Seijin humans and other species in Earth territory that do not learn how to control their abilities are mostly unstable or become insane. Therefore they need guidance. In addition to that, unregistered Seijins could do severe damage to society by using their abilities for criminal reasons.
Under Earth Federation law, all Earth Federation seijins have to be registered by the Guardians. After that, they have to choices: Work for the Guardians or take drugs to suppress their abilities. Most choose to work for the Guardians, because one common side effect of the drugs is severe depression which often ends in suicide.
When the Earth Federation decided to dissolve the Guardians after discovering their inhuman methods, the Guardians decided to take over the earth, only to be defeated by the Earth Federation forces and rogue Seijins. As a result, the Guardians were disbanded and those guilty of war crimes prosecuted and rendered powerless through the use of advanced Seijins. Their organization was replaced with the Seijin Monitoring Commission.