Kakugan

A kakugan (赫眼, red eye) denotes a ghoul's eye where the iris turns red, and the sclera black with red veins across the eye and into the skin around the eye. The ghoul can enter this state by their own will, but this state is also entered when the ghoul becomes excited, gets extremely hungry, or when they use their kagune. A kakugan is an influence of activated Rc cells.

Generally, a ghoul possesses two kakugan and they always activate together. If a subject possesses only one kakugan, they are a one-eyed ghoul. A new type of ghoul/human hybrid called a Quinx possess only one kakugan like a one-eyed ghoul, but their sclera is grey, while in the anime it is black. It is unknown if the kakugan serves any supplemental purpose. All members of Legion had their kakugans activated in both eyes.