IBM, or Invisible Black Matter, is not an individual character but rather a significant entity within the 'Ajin' series. It is a substance that manifests as a shadowy humanoid figure controlled by Ajin, humans who cannot die. These entities serve as both protectors and weapons for Ajin, capable of performing various tasks, including combat and surveillance. The IBM plays a crucial role in the series, highlighting themes of immortality, fear, and the moral complexities of using such powerful entities for personal gain or protection.