Originally born as a human, his encounter with a female ghoul under the name of Kamishiro Rize forced him to undergo emergency surgery procedures after his assault by her. Cunning and manipulative, Rize uses her charms and Kaneki’s attraction toward her as a means of getting her next “meal.” After the attack, Kaneki is rushed to the hospital in a half-dead state with Rize. A doctor of the name Kanou, who secretly experiments with ghouls, humans, and half-ghouls, transplants what Kaneki believes to be a kidney into his body as a means of saving his life in the dire situation. Soon he learns that he had part of Rize’s kagune transplanted into him, becoming the first of many human-ghoul experiments Kanou will conduct.
Due to the unnatural abundance of RC Cells now present in his body because of Rize’s kagune, Kaneki loses the ability to stomach human food, often getting sick or vomiting when ingesting any, and gains the powers of regeneration and kagune usage as a result. Because of his morals and attachment to humanity, Kaneki refuses to consume human beings as a means to sustain himself, even when the meat is obtained through non-violent means, rather he consumes large amounts of coffee, and, when absolutely necessary, will cannibalize other ghouls.
Later, he joins with several ghouls, Banjou and Tsukiyama Shuu, in order to reduce the number of ghouls that cause havoc in the world to make the lives of humans better. Due to the nature of his ghoul mask and physical features, Kaneki earns himself the name “One-Eye” among the ghoul-community, and eventually “Centipede” when his kagune takes the form of one after he is put under immense stress, pressure, and anxiety. The CCG eventually dub him to be an SS-Rated ghoul.
During the time he is being tortured by Yamori, Kaneki’s hair turns white and his nails turn black because of the stress and repeated mutilations of his fingers and toes.
When a ghoul cannibalizes another ghoul, some of their RC Cells are absorbed into the consuming ghouls body. As a result, said ghoul will eventually lose himself to insanity and madness because of the overabundance of RC Cells, even for ghouls. Yamori (Jason) is a prime example of cannibalizing ghouls and becoming a victim of insanity, eventually Kaneki will, too. Kaneki’s centipede state is a perfect example of his fight with losing himself to the RC Cells and memories of other ghouls.
After surviving his battle against Arima, Kaneki loses his memories of his past and becomes unaware of his true self.
Kaneki now lives under the new identity of Haise Sasaki and has been taken into the care of the CCG. He is a Associate Special Class Ghoul Investigator, the former mentor of the Quinx Squad and a member of Team Mado. Kaneki’s duties are overseen by Akira Mado and Kishou Arima.
During his fight with Kanae, Kaneki regained his memories. With the return of his memories, Kaneki has become emotionally cold and detached. It’s revealed that during his fight with Arima, Kaneki wished to die. This desire has remained in Kaneki. His cold demeanor is a result of Kaneki’s desire to die.