The Zero Unit is a mysterious symbol that is often identified with the mythological "Keter" of the Sephirot. The symbol takes the shape of one large triangle formed from four smaller ones. Inverted Zero Units also appear on Guilmon and BlackGuilmon's snouts.
The Zero Unit on Calumon's forehead shines whenever he releases the light of Digivolution.
List of Digimon with the Zero Unit