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. By using airborne-transmitted diseases like those utilized by Furuta Washuu, the condition known as ROS (an overload of RC cells) can lead to artificial ghoulification.
Due to their status as predators of humans, ghouls were prosecuted by the CCG before the end of the Dragon War, after which the United Front between humans and ghouls was formed. As a result of this and the development of synthetic food, ghouls were allowed to co-exist with humans. However, some ghouls went rogue, refusing to adapt to this new lifestyle.
In addition, it was predicted by Kishou Arima that at some stage every human would eventually become a ghoul, though this prediction has yet to be proven correct even after the final chapter.
During the Dragon event, it is theorized that the Ghoul race came to be hundreds of years in the past after an unknown individual first underwent an explosive reproduction of RC cells and experienced a Dragon-like transformation, releasing spores that mutated a number of humans into ghouls through ROS.
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 attempt to eat normal food, they will be struck by a powerful urge to vomit. When forced or forcing themselves to digest such food, their physical condition will deteriorate. While ghouls cannot eat normal food, they are, however, able to drink coffee(and eat the beans) , as well as regular drinking water.
However, after the unification of ghouls and humans after the Dragon War, a synthetic food was developed by scientists including Kimi Nishino, though it was not revealed how much was needed to sustain a ghoul, only that even after modification it tasted disgusting.
Ghouls do not need to eat regularly like humans do. They can survive for one or two months on just 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 their mental ability will be impaired, 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 shown when Kaneki was willing 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".
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 struck with enough force, and can penetrate a human's body with their bare hands. They are also capable of jumping to heights not achievable by a regular human. Their bodies are extremely resistant to injury. For example, if a ghoul were to be stabbed with a knife, the blade would break instead, suffering a small scratch that heals almost instantly. However, higher kinetic forces, such as a fall from a great height, can and will harm them.
Ghoul senses are much keener than those of 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 in general is unknown. At 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 that of 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, tentacle-like appendages, a tail, or a blade-like object, and can even be detached from the ghoul, depending on the ghoul's materials and his or her own Rc type. It is possible for a ghoul to develop another, typically 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 when using their kagune.
Ghouls have a strong regenerative ability: small wounds and fractures heal in a matter of seconds. Larger wounds take longer to heal, but will usually vanish after a few days, as long as the ghoul has access to 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 alongside 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 from a great height.
Ghouls had two choices of lifestyle: living as hermits; isolated and avoiding society's eyes, or attempt to assimilate into human society. However, after the unification, co-existence was promoted. Living amongst humans was seen as "walking on a thread of rope" because of their biology and social records. Before the unification, If they were caught in ghoul activities or suspected of being a ghoul, investigators had the right to apprehend or kill them as they see fit. Ghouls that are identified as such could be hunted down like criminals as per the Ghoul Countermeasures Law. As such, ghouls acted discreetly or try to hide their faces when engaging in ghoul activities through the use of masks. Ghouls in Tokyo, in particular, had 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 solely on their skills of 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 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.
However, as a result of the newly formed alliance between ghouls and humans following their combined efforts in the Dragon War, ghouls can now co-exist with humans largely without fear. Their feeding needs are met by synthetic food developed in labs.
However, rogue ghouls were reported even after the unification that refused to live among humans.
- A one-eyed ghoul is a rare human-ghoul hybrid, either created artificially or naturally born between inter-species relationships.
- A Quinx is a variation of artificial one-eyed ghoul created by the CCG with restrained kakuhou through the use of quinque steel.
- An Oggai is another variation of artificially created one-eyed ghoul, also created by the CCG through the use of Rize Kamishiro's kakuhou. However, after the destruction of all 100 Oggai and the death of Rize Kamishiro, creating any more Kamishiro Oggai would be near impossible.
- A half-human is a human-ghoul hybrid born without ghoul abilities.
- A modified human is a human who was a failed one-eyed ghoul experiment by Dr Kanou that are often distorted, muscular, mentally disabled and lacking of ghoul abilities, they are often given out as scrappers.
- A kakuja is a ghoul whose Rc cells have mutated and developed an armor-like kagune through repeated cannibalism.
- In the last chapter of :re, Kuki Urie states that ghouls may have come into existence through a dragon such as those formed by the Underground King and Ken Kaneki.