Ghoul

Ghouls (, gūru, translates approximately to eater species) are a carnivorous and cannibalistic species that are 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 half-human), and through a successful kakuhou transplant surgery, an artificial hybrid can be created.

Due to their status as predators of humans, ghouls are prosecuted by the CCG.

Diet
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 is also different from humans', making other foods taste disgusting and uncomfortable. 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. While ghouls cannot eat normal food, they are able to drink coffee. They are also able to drink plain water.

Ghouls do not 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, as seen when Kaneki was about to eat Hideyoshi Nagachika, his best friend, before being stopped by Touka Kirishima. She describes this state of near-starvation as hell for any ghoul.

Biology
Ghouls have a high physical capability and are four to seven times 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. They also have a tough body that makes them resistant to injuries. For example, if a ghoul were to be stabbed with a knife, the blade of the knife would break instead, the ghoul only suffering a small scratch that heals almost instantly. However, forces that are strong enough can break their tough body, such as a fall from an extreme height.

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

The origin of these aspects of ghoul biology and the origin of ghouls is unknown. By and large, nothing is currently known of the origin of the ghoul species, except that Rc cells play a huge part in their 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, or even detached from the ghoul, depending on the ghoul's materials and his or her own Rc type. It is possible for a ghoul to have developed another, usually armor-like, kagune as part of the mutation caused by excessive cannibalism. Ghouls also have an identifying feature known as a kakugan, an affliction manifesting in the eyes as red irises surrounded by a black sclera. A ghoul's kakugan can appear either by their own will, due to hunger or from using their kagune.

Ghouls have a high regenerative ability: Small wounds and fractures heal in a matter of seconds to a day. Larger wounds take longer to heal, but will usually heal after a few days, as long as the ghoul has an adequate food supply. Ghouls have been shown to regain their strength and heal wounds almost immediately after consuming a small amount of flesh, suggesting that they can quickly metabolize their food to boost their regeneration. This regenerative ability varies among individuals, with some ghouls exhibiting the ability to reattach severed body parts or regrow detached limbs. In extreme cases of regenerative ability, some powerful ghouls can even heal severe brain injuries, reattach their severed head or upper body, and regrow large sections of their bodies.

Ghouls prefer to consume human flesh, but it is not uncommon for them to cannibalize one another. Ghoul tissue is considered coarse and disgusting, but it has the merits of providing an alternative food source. Furthermore, cannibalism increases a ghoul's Rc levels to the point of mutation, increasing their kagune's overall power. Occasionally, cannibalistic ghouls gain through this mutation an armor-like kagune besides the predatory kagune that ghouls commonly possess, turning them into kakujas.

In extremely rare cases, a ghoul whose parents possess different Rc types will inherit both from their parents. This results in a ghoul that possesses a kakuhou in more than one location and is able to use two different types of kagune, a chimera kagune.

Though powerful and durable predators, ghouls are still far from immortal. Despite their superior physical status, ghouls age at the same rate as humans. All ghouls, though resistant to damage from man-made objects, are vulnerable to damage or modifications made to their mucous membranes, such as nostrils, eyes, ears, and lips. Generally, a ghoul will die if it suffers massive trauma that proves fatal before its innate healing ability can stabilize its condition. The most damage that can be inflicted to a ghoul usually comes from other ghouls, as they have the physical strength and the use of their kagune that can deal heavy damage to a ghoul. However, they are also vulnerable to quinques and Q bullets, which can injure a ghoul effectively due to the nature of such weapons as products of a harvested kagune. They can still suffer injury from an acute transfer of a sufficient volume of kinetic energy, such as a heavy high falling object or jumping down a great height. Ghouls become vulnerable if forced to consume a sufficient amount of normal food, which will weaken their physical condition and possibly makes them vulnerable to man-made objects such as guns, knives, swords, etc. Rc suppressants have a similar effect, as their administration make ghouls as vulnerable as humans.

Lifestyle
Ghouls have two choices of lifestyle: living as hermits; isolated and avoiding society's eyes, or 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 as per the Ghoul Countermeasures Law. As such, ghouls will act discreetly or try to hide their faces when engaging in ghoul activities with masks. Ghouls in Tokyo, in particular, have developed a trend of wearing them to hide their faces, making the mask the identifying feature instead of their face, some even forming gangs based on their masked identity. Since the ghouls are only identifiable by their masks, they are safe so long as their face is not exposed. Others like Rize rely on their deception and charm to win over their prey. Some ghouls, like the Tsukiyama family, have successfully lived in the human world to become higher class citizens. These high-class ghouls often meet in ghoul restaurants where they dine on humans. Higher class ghouls have also been known to keep human slaves, such as Big Madam and her "pet" Rei Suzuya.

Some ghouls prefer to live as hermits instead. Because a ghoul does not 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 just as uncomfortable as it might be for a human. Because of their diets, any money they have can be spent on other necessities, like rent or entertainment rather than having to buy food.

Ghouls 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 afterward, or they will 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. Places such as Anteiku can provide human meat for these ghouls. 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 rare human-ghoul hybrid, either created artificially or naturally born between inter-species relationships.
 * A half-human is a born human-ghoul hybrid without ghoul abilities.
 * A kakuja is a ghoul whose Rc cells have mutated and developed an armor-like kagune through repeated cannibalism.

Trivia

 * Ghouls can smoke as well as have allergies to metals, plastics, and other materials.