Haise Sasaki

Haise Sasaki (佐々木 琲世, Sasaki Haise) is the main protagonist of Tokyo Ghoul: RE. He is a Rank 1 Ghoul Investigator, the mentor of the Quinx Squad and a member of Team Mado. Haise's duties are overseen by Akira Mado and Kishou Arima.

Appearance
Haise is a slim, pale-skinned young man with slightly messy, white hair adorned by black streaks. He has grey eyes, wields one kakugan in his left eye; which is an indication that he is a one-eyed ghoul, possibly originating from artificial means. Similar to the other Ghoul Investigators, Haise is seen well dressed in the standard suit and tie most of the time. When embarking on a mission, he wears a white trench coat over his uniform and occasionally carries a briefcase with him that contains a death note.

He is commonly said to resemble Ken Kaneki due to his physical appearance and personality, and the fact that their physical information align. Some fans have theorised that Kaneki was brainwashed and was forced to live with a new name. It is highly assumed that he is, though no such information is revealed in the manga as of yet.

Personality
Haise is a self-contained, good-natured individual. As a Ghoul Investigator, he comes off as trustworthy, loyal and devoted. While he portrays a serious demeanor towards his work, Haise is also merciful to some extent, as he believes that an Investigator should not annihilate ghouls unnecessarily. Though he is hailed as the mentor of Quinx, he shows apportioned respect and great concern for his colleagues, pledging a protective streak over them. He also complies with the obligations assigned by his seniors without hesitation. Haise habitually reads books, and is very co-operative, as he’d prefer to work with his group than to single-handedly accomplish the CCG’s goals. It was revealed that Haise likes to do puns, shown by his conversation with Akira Mado in the naan restaurant. Although it was not confirmed yet, Haise has shown his habit to put his hand on his right ear/scratching his head whenever he is thinking hard or feeling uncomfortable (which is very similar to Kaneki's behaviour when he is hiding something).

Theory on Sasaki being Kaneki
(Since it's extremely important to include this information)

It has become a widespread theory throughout Tokyo Ghoul readers that Sasaki is also Kaneki Ken, especially since their details and information match up smoothly. TG:Re occurs three years after the end of Tokyo Ghoul, meaning that Kaneki would've been 22 by the time TG:re was taking place (take note that Sasaki is also 22). Sasaki and Kaneki share a 1cm difference in terms of height, and that they also have a 3kg difference in terms of weight (none of these factors are a surprise as well). Their hair style and colour are also similar with one another, and the fact that they are both one-eyed ghouls can be a factor as well.

Brainwashed: Most readers speculate that Kaneki was either brainwashed by CCG, or received major memory loss due to his fight with Arima. It was then that Kaneki was taken in by CCG and trained under a new name and identity.

Kaneki/Sasaki as a CCG weapon: It is also possible that CCG/Arima wanted Kaneki to stay alive for various reasons. One main reason would be the fact that Kaneki was able to lay a finger on The Owl whilst Arima wasn't able to, which may have gotten their attention. They might be wanting to use Kaneki as a weapon for defeating The Owl, and the entire "memory loss" theory can also be advantageous since Kaneki/Sasaki wouldn't find a reason to defy CCG since it would seem to him that they're the only ones who know about him. This might be the reason why CCG said through Akira that they wanted an investigator who was stronger than Arima (most likely to be Sasaki).

The white figure at the end of Chapter 143: It is not known who the figure at the very end of the manga was, but people believe to be either Haise or Arima since the figure also had a briefcase with him (which might've been the briefcase used for Quinques). If it had been Haise, then there is the new possibility that Haise (if he was Kaneki) still retained his memories, but decided to keep his status a secret from Anteiku and his other friends. If it had been Arima, then he might've simply been watching over Touka in order to find out if they were aware of Sasaki/Kaneki's state or not.

Urie Kuki: Some people also believe that Kaneki might be Kuki rather than Sasaki, since they also look slightly similar, though neither of their details properly match up.

Kaneki isn't Sasaki: Kaneki and Sasaki may also not be entirely connected at all. Some believe that Kaneki might reappear in the later chapters of TG:re, either as an ally or an enemy of Quinx. There is also the sad possibility that Kaneki really is dead and everyone's just making up theories in order to feel better whilst the entire fandom cries in denial.