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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.
One-eyed ghouls earn their name from the single kakugan they possess, in contrast to normal ghouls. 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 Yoshimura 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.
Rumors state that hybrids are more powerful than ordinary ghouls due to hybrid vigor, though it has not been further elaborated on. Most specimens have demonstrated radical evolution of their abilities and developed partial to full 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. Dr. Shiba claims that the more half-ghouls and Quinx get hurt, the more their basic abilities will increase.
Artificial one-eyed ghouls can only consume flesh of human or ghoul origin like normal ghouls and thus they are strictly a cannibalistic organism. However, natural one-eyed ghouls like Eto are able to taste and consume human food without negative effects.
Hybrids come into existence through two methods, either normally born as a result of the union between a ghoul and a human or an artificial half-ghoul, or through transplanting a kakuhou into a human host.
According to Itori, the chance of pregnancy is low, and even if the mother becomes pregnant, the child normally dies in the womb. Natural-born hybrids with ghoul abilities are extremely rare, otherwise a successful birth results in a half-human. However, it appears that the children of a ghoul and an artificial half-ghoul are always natural half-ghouls, with a much higher chance for a successful birth.
- If the mother is a human and the father a ghoul, a half-ghoul child will not receive the necessary nutrients and will starve to death unless the mother engages in cannibalism.
- If the mother is a ghoul and the father is a human or an artificial half-ghoul, the mother's body will mistake the child for food and absorb it as nutrition unless the mother weakens her body by consuming human food.
- Main article: Kakuhou transplant surgery
The primary method of creating one-eyed ghouls is through transplanting a kakuhou into a human or half-human recipient, an exceptionally high-risk procedure. In the rare event that the operation is successful, the 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.
Dr. Kanou, while experimenting with Seidou 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. However, it was confirmed that in some less successful cases the process also shortens the subject's lifespan.
- Eto Yoshimura (human mother, ghoul father)
- Ichika Kaneki (ghoul mother, one-eyed ghoul father)
- Underground King
- Ken Kaneki/Haise Sasaki (K-Eater)
- Kurona Yasuhisa (B-Eater, Floppy)
- Nashiro Yasuhisa † (W-Eater, Floppy)
- Kichimura Washuu †
- Kanou's numerous failed experimental subjects (Floppies)
- Main article: Half-human
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 kagune, kakugan).
- Main article: 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.
- Main article: Oggai
Oggai are stated to be made differently from the Quinx in a new and innovative process. They were created under the authority of newly-appointed bureau director Kichimura Washuu using Dr. 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 were all based.
So called "modified humans" are distorted failed experiments of Dr. Kanou that did not quite become one-eyed ghouls (hence their occasional need for weapons like quinque). 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 as scrappers for the Ghoul Restaurant.
- Eto Yoshimura and Ichika Kaneki are the only confirmed natural-born half-ghouls.
- Fujishige Iba claims 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.