A Kakugan denotes a ghoul's eye where the pupil 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.
Masks are worn by ghouls to prevent their identities as 'humans' from being discovered by the CCG. Some of the masks also bear a sort of symbolic attachment, or representation. An example would be Kaneki's mask, which bears a Frankenstein motif. The design is meant to reflect his status as one who exists in the world of Ghouls and in the world of humans. Touka's mask stems from her affection towards rabbits.
Ghouls are rated based upon various factors, including their basic strength, activity level, and how hostile they are towards Investigators. There are six rating levels, from SSS (the most powerful) to C (the weak). In some cases, an "~" may be added to the base rating. This indicates that the Ghoul is estimated to be at least that strong, but the rating may change once more is known about the subject.
The rating of a ghoul is set through evaluation of the ghoul's individual ability. The following criteria are employed to determine the rating according to their individual ability:
The criteria for SSS ratings are extremely dangerous, needing to be dealt with utmost caution.
- Multiple Special Class Investigators are needed to deal with an SS-rated ghoul.
- The ability of an S+-rated ghoul is (appears to be) equivalent to the ability of an average Special Class Investigator.
- The ability of an S--rated ghoul is (appears to be) equivalent to the ability of an average Associate Special Class Investigator.
- An A-rated ghoul is comparable to a First Class Investigator.
- A B-rated ghoul is comparable to a lower-ranked Investigator (Rank 1 to Rank 3).
- A ghoul of smaller ability is C-rated.
In addition to the ghoul's individual ability, the actual damage caused by the ghoul is also evaluated. The rating is set in consideration of the predation and killing incidents committed by the ghoul. For some ghouls their influence over other ghouls is also taken into account when ratings are given.
Besides water and coffee, a ghoul can only feed on humans and other ghouls. The structure of their tongues are also different from humans', making other foods taste disgusting and uncomfortable. While ghouls can't eat normal food, they are able to drink coffee. They are also able to drink plain water. If ghouls try to eat normal food, they will be struck by a powerful urge to vomit. When forced or forcing themselves to digest such food, it will degrade their physical condition. Ghouls don't need to eat in the short term like humans do. They can survive for one or two months just feeding on one body. However, some ghouls eat merely for pleasure. When a ghoul enters an extreme state of hunger, they will suffer very painful headaches and lose all reason, causing them to become driven by instinct alone. They will feed on any available source of human meat to end this state, even that of close friends. Although ghouls can go up to two months without human flesh, some ghouls can feed on more than eleven people in just a few days, This is known as 'binge eating.'
NEW: Some ghouls, that see eating humans is mad-like, tend to eat wild animals to keep themselves alive. However, they need to eat at least one human in two months.
NEW: Some ghouls can have mutations, that affect their physical strength. These mutations can start from kakugans to kagunes, not calling them kakujas. However, these ghouls are seen as a target for other ghouls, as they are nothing more but mutated 'rats'.[/color]