Dr. Cain is a scientist who discovered Dr. Light's laboratory and Megaman X. He used X as a template to create similar robots to help humanity that became known as Reploids. As he did not fully understand Light's technology, some Reploids became rogues known as Mavericks.
To stop the Mavericks, Cain created Sigma to lead the Maverick Hunters. Unfortunately, Sigma himself defected as well and became a great threat to the world.