A mutant is someone who suffers from a biological mutation that changes them on a genetic level. The chemicals responsible for causing mutations are mutagens, with most being carcinogenic (cancerous) in nature. Fictional mutants tend to have mutations that go far beyond what is typically seen in biological mutants, and often result in the mutated life form exhibiting superhuman abilities or qualities.
Mutants live in the Roppongi section of the Underworld. They were cast away and sealed below by the Center because of the mutations they suffered thanks to the Nuclear War. They desire to return to the surface to see the sun once more.