One-eyed ghoul

A one-eyed ghoul (隻眼の喰種, sekigan no gūru), also called half-ghoul (半喰種, han-gūru) is a human-ghoul hybrid who possesses a ghoul's abilities. Most ghouls treat their existence as an urban legend or have never heard of their existence at all.

Characteristics
One-eyed ghouls earn their name from the single kakugan they possess, in contrast to normal ghouls. In all other ways, they appear to have no obvious differences from an ordinary ghoul. Some individuals are noted to have a unique scent that combines traits of a human and a ghoul, identifying them as different and often considered "enticing" by certain ghouls. Ken Kaneki has been shown to smell like a ghoul by some individuals whereas some cannot tell the difference, and Eto has been shown to smell like a human so far. It is unknown whether all hybrids have this trait. Their flesh and blood is also noted to be particularly delicious, making them a potential target for cannibalism. Dr. Shiba claims that the more half-ghouls and Quinx get hurt, the more their basic abilities will increase. Rumors state that hybrids are more powerful than ordinary ghouls due to hybrid vigor, though it has not been further elaborated on. All specimens (except Furuta and Nashiro) have demonstrated radical evolution of their abilities and developed into, sometimes partial, kakujas within a short period of time. Physically, their bodies are identical to that of a ghoul with the same resistance to harm and ability to use a kagune. Like normal ghouls, they are only able to consume meat of human or ghoul origin, as well as coffee and water; Eto has been shown consuming human food, but it is unknown if she can eat it without any adverse effects at this time. As such, they are inherently a cannibalistic organism. Hybrids come into existence through two methods, either normally born as a result of the union between a male ghoul and a female human, or through transplanting a kakuhou into a human host.

Pregnancy
Naturally-born hybrids born with ghoul abilities are extremely rare. According to Itori, the chance of pregnancy is low, and even if the mother becomes pregnant, the child normally dies during pregnancy.
 * If the mother is a human and the father a ghoul, the child will not receive the necessary nutrients and will starve to death unless the mother engages in cannibalism.
 * This only applies if every hybrid child resulted in a one-eyed ghoul, which, according to Arima, the chances are far too low. Indicating Itori is possibly unaware of the existence of half-humans.
 * If the mother is a ghoul and the father is a human, the mother's body will mistake the child for food and absorb it as nutrition.

Kakuhou Transplant
The primary method of creating one-eyed ghouls is through transplanting a kakuhou into a human recipient, an exceptionally high-risk procedure. In the rare event that the operation is successful, the human patient develops into an artificial hybrid with all the abilities of a normal ghoul. This procedure was pioneered by Dr. Akihiro Kanou, with the first successful patient believed to be Ken Kaneki. Kanou, while experimenting with Takizawa, compared his surgery with grafting, a horticultural technique where the tissue of two trees are combined to help them both grow stronger. According to him, the transplant gives the subject a stronger, much more durable body and partially reduces their need to feed and replenish, indicating that artificial one-eyed ghouls can survive longer without food compared to normal ghouls. Although artificial eye-only ghouls can live longer than humans and the Human Middle they must intake more RC cells because if they do not feed for too long, their lifespan will be slightly "shortened" and if the afflicted individual continues to use the kagune their situation could worsen. There are two solutions to slow this aging and to regain strength: the first is to not use the kagune and the second is to practice cannibalism by eating other ghouls.

Natural

 * Eto Yoshimura
 * Underground King

Artificial
Eater-series/Rize-based Owl-series/Yoshimura-based
 * Ken Kaneki/Haise Sasaki (K-Eater)
 * Kurona Yasuhisa (B-Eater, Floppy)
 * (W-Eater, Floppy)
 * Kichimura Washuu
 * Kanou's numerous failed experimental subjects (Floppies)
 * Seidou Takizawa (T-Owl)
 * Koutarou Amon (A-Owl, Floppy)

Fictional

 * Nameless

Half-humans
Should a child be conceived from a ghoul father and a human mother, there are mostly likely to be born as half-humans and are far more common. They possess enhanced physical abilities but not the traits from their partial ghoul descent (i.e, lacking of kagune, kakugan).

Quinx
After examining Dr. Kanou's surgery on Ken Kaneki, the CCG created an experimental squad consisting of humans with ghoul capabilities through implementing a kakuhou covered by quinque steel layers. While the Quinx are not considered as normal half-ghouls due to their lower Rc cell levels and the ability to eat human food, they are made from the concepts of Dr. Kanou's half-ghoul experiments.

Oggai
Oggai are stated to be made differently from the QS in a new and innovative process. They were created under the authority of newly-appointed bureau director Kichimura Washuu using Akihiro Kanou's perfected kakuhou transplant surgery. As a new race of one-eyed ghouls Oggai have the previously unheard of ability to choose either to activate one or two kakugan at once. It is estimated nearly a hundred individuals were produced, each enhanced to be superior trackers and adept ghoul exterminators. Kichimura compares them to Ken Kaneki, the original prototype on which they are all based.

Modified Humans
So called "Modified Humans" are distorted failed experiments of Kanou that did not quite exactly become One-Eyed Ghouls (hence their need to use weapons like Quinques at times) like Taro. They tend to be intellectually disabled, massive in size, and muscularly deformed. The majority of the failed experiments that fall into this category were given to Madame A as pets/her "children" or scrappers.

Trivia

 * Eto is the only confirmed case of a natural-born half-ghoul.
 * Fujishige Iba once mentioned to his subordinates that there was a scientist that wanted to experiment with hybridization, but his research was deemed unethical and shut down.
 * Akihiro Kanou refers to the failures from his experiments as floppies, though it unclear what difference there may be between a floppy and an artificial one-eyed ghoul. However, it could be that they are not as strong as he hoped. This was shown when he compared the Yasuhisa twins to Kaneki he mentioned that they were not as strong as him thus they are failures, thus floppies.
 * According to Eto, about a hundred years prior to the current timeline and events in the manga series, there existed a one-eyed ghoul so powerful that the ghoul's appearance led to the foundation of the CCG's predecessor organization.
 * Kurona Yasuhisa and Kuki Urie have both demonstrated the ability to manifest kakugan in both eyes. It is unknown what causes this to occur, or what it means in relation to other artificial hybrids.