The mutants the group encounters are humanoid monsters with glowing and pulsing metal brains. These monsters are mostly humans who have been mutated by biotechnology created by RAPT under the direction of the ZERO Organization. The leader of the Zero Organization is Glenford and is responsible for most of the projects that Jo and her friends had come across. The things that Glenford was involved with includes the prefectural governor, RAPT itself, the pharmaceutical company, and the metal brain incidents.
When Jo came into contact with a metal brain, it showed her images of her past as one of the three Genocide Angels created by the RAPT.