A mutant is a being whose physiology has been altered by the radiation and pollution that occurs in the post-apocalyptic world that the D series takes place in. In addition to an altered appearance, they sometimes gain unique powers which give them advantages over others. Characters in the series who are mutants include:
- Rei-Ginsei, a human-looking mutant with space alteration powers.
- Others in the service of Count Magnus Lee who attempt to hinder D.
- Many members of the Barbarois, notably Mashira, a mutated werewolf and his companions Benge and Caroline