Haise Sasaki is the identity assumed by Ken Kaneki following a near-fatal confrontation with the premier ghoul investigator, Kishou Arima. During the raid on the Anteiku coffee shop, Kaneki was brutally defeated by Arima, suffering catastrophic injuries that included severe trauma to his head. While he survived the encounter, the damage caused a complete loss of his memory. Arima took the amnesiac Kaneki into the custody of the Commission of Counter Ghoul, where he was eventually rehabilitated and given a new name. The name Haise is derived from two kanji characters that Kenji had favored, meaning coffee and world. In this new life, Haise Sasaki becomes a First Class Ghoul Investigator, working alongside the man who once defeated him.

In stark contrast to the tortured and often angst-ridden personality of Ken Kaneki, Haise Sasaki is defined by his gentle, amiable, and cooperative nature. He is portrayed as a kind and patient mentor who is highly sociable and gets along well with his colleagues. This persona is notably more stable and cheerful than his previous identities, often displaying anxious mannerisms such as scratching his head and even using puns. However, this calm exterior is fragile. Haise suffers from frequent hallucinations and vivid dreams, which are in fact fragmented memories of his past life as Ken Kaneki. He experiences internal conflict as the personality of Kaneki, often appearing as an illusion or a child, tries to resurface and take control. Consequently, Haise lives in fear of fully remembering his past and rejecting his true ghoul nature, desperately trying to suppress the powerful identity lurking within him.

As a ghoul investigator, Haise Sasaki is primarily motivated by his duty to the CCG and his paternal dedication to the Quinx Squad. He functions as the mentor and supervisor of this experimental unit, treating its young members as if they were his own children. His mission is to guide them in controlling their unique abilities and to lead them in the eradication of ghoul threats. Despite his loyalty to the CCG, his actions are subconsciously influenced by his buried past. He shows mercy to ghouls and is instinctively drawn to his old life, such as when he visits a coffee shop that evokes powerful, unexplained emotions in him. His primary internal motivation is to maintain his identity as Haise and prevent the resurgence of Kaneki, a battle that becomes increasingly difficult as fragments of his memory return.

Within the narrative, Haise Sasaki serves as the protagonist and the central figure of the Quinx Squad. His role is to lead this controversial team of human-ghoul hybrids on missions against dangerous ghoul organizations. His position in the CCG places him in direct opposition to his former allies from Anteiku, creating profound dramatic irony. As a special operative mentored by Arima and observed by Akira Mado, Haise is a key asset in the CCG’s operations, yet his stability is critical; if he remembers his past, he could become their greatest enemy. The central tension of his role is the duality of his existence—fighting ghouls as a human while secretly being the very thing he is sworn to destroy.

Haise Sasaki forms critical relationships that act as a surrogate family structure, replacing the connections Kaneki lost. He views Kishou Arima as a paternal figure, the one who gave him a second chance at life and serves as his mentor. Akira Mado takes on a maternal role in his life, watching over him and the Quinx Squad. His most defining relationships are with the members of the Quinx Squad: Kuki Urie, Ginshi Shirazu, Saiko Yonebayashi, and Tooru Mutsuki. He acts as their fatherly leader, responsible for their training, safety, and emotional well-being, often calling them his children. Outside the squad, he maintains a friendly rapport with other investigators, such as the eccentric Juuzou Suzuya, whom he often gives sweets.

The character development of Haise Sasaki is defined by his gradual psychological deterioration and eventual reclamation of his original identity. For a significant period, he successfully functions as a mentor, but the stress of combat and encounters with figures from Kaneki’s past cause his suppressed memories to surface. During the Tsukiyama Family Extermination Operation, he is pushed to the brink of death. In this desperate moment, the fragment of Ken Kaneki within him forces a final confrontation. Accepting that he cannot erase Kaneki, Haise willingly steps aside, allowing the original personality to reawaken. The emergence is not a simple return to the old Kaneki; Haise first transforms into a cold, ruthless persona known as the Black Reaper, characterized by his black hair and a nihilistic, efficient approach to combat. This persona bridges the gap between Haise's idealism and Kaneki's survivor instincts.

As an artificial one-eyed ghoul, Haise Sasaki possesses formidable abilities, though his amnesia prevents him from fully understanding or accepting them. His primary weapon is a rinkaku kagune, which manifests as a set of four to six tentacle-like appendages from his lower back. He also retains the accelerated regenerative healing factor typical of ghouls, allowing him to recover from severe wounds that would kill a normal human. Despite being a ghoul, he operates as an investigator, able to use a quinque (a weapon made from a ghoul's kakuhou) in addition to his own predatory organ. As his psyche fractures and he embraces his true nature, his power escalates. He develops a kakuja, a more powerful and armor-like form of kagune created by cannibalizing other ghouls. This culminates in his transformation into the Dragon, a city-sized, uncontrollable kakuja entity of immense destructive power.