Ghoul

Ghouls (喰種, gūru, translates approximately to eater species) are a carnivorous and cannibalistic species that is only able to feed on humans and other ghouls. They are as close to humans as possible: They normally have the same physical appearance and intelligence as a human with the exception of diet, mentality and inner biology. If certain rare conditions are met, ghouls and humans may even be able to have offspring (see One-Eyed Ghoul), and a ghoul's organs can be successfully transplanted into a human.

Due to their feeding habits, ghouls are prosecuted by the CCG.

Feeding Habits
Besides water and coffee, a ghoul can only feed on humans and other ghouls. They are unable to digest any other type of food due to a particular enzyme their bodies produce. 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. An example would be Shuu Tsukiyama, who is fixated upon eating Ken Kaneki due to a bizarre curiosity as to how half-ghoul flesh would taste.

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. Touka Kirishima describes this state of near-starvation as hell.

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,' a habit sustained by Rize Kamishiro.

Physical Characteristics
Ghouls have high physical capability and are much stronger than the average human: They are able to send the average human flying if they strike with enough force and can penetrate a human's body with their bare hands. They are also able to jump several meters high.

Under normal conditions, ghouls have a high regenerative ability: Small wounds and fractures heal in a matter of hours to a day, and even grave wounds heal after few days, as long as the ghoul has adequate food supply. They also have a tougher body than most regular solid materials. For example, if a ghoul were to be stabbed with a knife, the metal of the knife would break. Ghouls' body wouldn't get injured easily and only more keen tougher materials can injure them. A strong enough force may also be able to break their tough body.

Ghouls have better senses than humans. They can smell people or meat from afar and can tell apart humans and ghouls by their smell. They also have a heightened sense of hearing that even allows some ghouls to discern individual footsteps from far away.

Ghouls seem to age at the same rate as humans, further allowing them to blend in.

Biology
Ghouls have an "Rc factor" ten times higher than a human. If the average healthy human has about 200-500 Rc level, then the ghoul has about 1000-8000 Rc level. They possess an organ known as the kakuhou, from which Rc cells harden and break through the skin to form the kagune. The kagune is a hard, muscle-like protrusion described as their claws. It can take the form of a pair of sharp wings or tentacle-like appendages, a tail, or a drill-like object, depending on the ghoul's materials and his or her own Rc type. It is possible for a ghoul to have develop another, usually armor-like, kagune as part of the mutation caused by eating another ghoul. Ghouls also have an identifying feature known as a Kakugan, an affliction manifesting in the eyes as red pupils surrounded by black sclera. A ghoul's kakugan can appear either by their own will, due to hunger or from using their kagune.

Ghouls have a liking for human meat, but it is not uncommon for them to cannibalize one another. Ghoul meat is considered coarse and disgusting but it has the merits of being an alternative food source and increases a ghoul's Rc levels to the point of mutation, evolving their kagune's overall strength. Occasionally, cannibalistic ghouls gain through this mutation an armor-like kagune besides the predatory kagune that ghouls commonly possess, turning them into kakujas.

All ghouls, though invincible to damage from man-made objects, are vulnerable to damage or modifications made to their mucous membranes, such as nostrils, ears, lips, as in Uta's case.

Currently, the origin of these traces in ghouls is unknown. There are no known links as to where ghouls evolved or came from.

Weakness
Ghouls can be killed if forced to consume normal food, which will weaken their physical condition and possibly make them vulnerable to man-made objects such as guns, knifes, swords, etc.

Ghouls will die if they receive extremely critical or life-threatening injuries that will kill them before they have a chance to heal.

It has also been said that a ghoul can be killed by decapitation but by what weapon or object is the mystery.

Lifestyle
Ghouls can live as hermits; isolated and avoiding society's eyes, or can attempt to assimilate with the humans. Living amongst humans is seen as "walking on a thread of rope" because of their biology and social records. If they are caught in ghoul activities or suspected of being a ghoul, investigators have the right to apprehend or kill them as they see fit. Ghouls that are identified as such can be hunted down like criminals.

Because a ghoul doesn't require much due to their high physique and their diet of human meat, they can live adequately as vagabonds with little to no money, although such a life is generally as uncomfortable as it might be for a human. Because of their diets any money earned at their jobs can be spent on other necessities, like rent or entertainment rather than having to buy food.

They can pretend to eat food in order to fool other people into believing they are human. However they must make themselves throw up soon enough afterwards, or they'll become sick. Because they can only consume human flesh, they are very territorial when it comes to "hunting grounds" on which it is appropriate to hunt humans without notice. Human consumption depends on the ghoul as some may prefer to be scavengers who only take from people who were recently deceased. Other ghouls may moderately hunt humans as carnivores with thoughts of appreciation for the lives they must consume, although there are ghouls who will indiscriminately devour as much as they can. Ghouls who indulge in excess have developed a superficial view on the value of life after consuming so many humans, much like how humans perceive livestock they grow and consume.

Variants

 * A One-Eyed Ghoul is a human/ghoul hybrid.
 * A Kakuja is a ghoul who has gained an armor-like kagune through repeated cannibalism.