Kōichi Sakakibara is portrayed as curious and determined, often seeking to uncover the truth despite repeated warnings from his classmates in Yomiyama North Middle School’s Class 3-3. His investigation into the events surrounding the class curse, including the disaster from 26 years earlier, highlights his persistence but also places him at risk. Early in the series, his curiosity leads him to suspect that Mei Misaki is a ghost, given that others refuse to acknowledge her presence. This same curiosity results in the class eventually treating him as “non-existent” as well, a situation he experiences with mixed feelings. Although he dislikes the isolation, it provides him relief in confirming Mei’s reality and strengthens their bond.

Kōichi is generally calm, friendly, and sociable, quickly establishing connections with his peers after transferring. However, he can be naïve and somewhat dense, often failing to recognize the feelings others have for him, such as those of Izumi Akazawa or Yukari Sakuragi, and occasionally responding in ways that alienate others. His lack of understanding extends to family dynamics, as shown by his inability to grasp the strained relationship between Mei and Yukiyo Misaki.

In his free time, Kōichi enjoys reading, particularly American horror fiction by authors such as Stephen King, H. P. Lovecraft, and John Saul. This interest earns him the nickname “Mr. Horror Lover” from Sanae Mizuno, while classmates give him other nicknames such as “Sakaki” (Naoya Teshigawara) and “Kō-chan” (Aya Ayano). He also expresses aspirations to attend a fine arts school to study carving and sculpting.

Although outwardly sociable, Kōichi is depicted as timid, a trait attributed in the novel to bullying he experienced at his former school due to sharing a surname with a serial killer. Despite his initial hesitance, he develops greater resolve as the story progresses, particularly in his determination to end the curse and protect Mei. This resolve is demonstrated when he stands against the entirety of Class 3-3 to defend her and later accepts the responsibility of confronting the “Extra” himself after discovering it to be his aunt, Reiko. His decision to kill her personally underscores both his commitment and the emotional burden he carries.

Kōichi also reveals a colder side of his personality in moments of high stress. When Izumi Akazawa is fatally wounded, she asks him to recall their first meeting. He bluntly tells her he does not remember, a remark she notes in disappointment before dying. His honesty, though harsh, is suggested to have been influenced by Izumi’s earlier attempt to harm Mei.

In the anime adaptation, Kōichi Sakakibara is depicted with a youthful face, short brown hair, and brown eyes, emphasizing his role as a newcomer to Class 3-3. In contrast, the manga portrays him with black hair and black eyes, giving him a slightly older appearance. His primary outfit is the standard Yomiyama North Middle School male uniform, which he wears throughout most of the series. Outside of school, Kōichi typically dresses in layered clothing, usually consisting of a T-shirt worn over a long-sleeved shirt, reflecting his casual but consistent style.

Kōichi Sakakibara is a 14-year-old boy who transfers from Tokyo to Yomiyama North Middle School in 1998. While living in Yomiyama, he stays with his maternal grandparents and his aunt, Reiko Mikami, as his father works abroad in India. His mother died shortly after giving birth to him, leaving Kōichi as the only child in the Sakakibara family.

As a transfer student, Kōichi is unfamiliar with the reputation of Class 3-3 and initially unaware of the curse that plagues it. His natural curiosity draws him to investigate the origins of the phenomenon, including the events that occurred 26 years earlier, despite repeated warnings from his classmates to avoid probing into the matter. His determination to understand the truth about the class curse becomes one of his defining traits and places him at the center of the unfolding mystery.

Mei Misaki
Kōichi first encounters Mei while hospitalized, meeting her in an elevator prior to his transfer to Yomiyama North Middle School. Once enrolled, he notices that Mei is isolated, with an older desk and no classmates acknowledging her. Despite repeated warnings to avoid her, Kōichi frequently approaches Mei and expresses concern for her well-being. Over time, the two grow closer, sharing conversations, walking home together, and confiding in one another. Kōichi demonstrates loyalty by defending Mei when Izumi Akazawa accuses her of causing the curse, and later protects her when other classmates attempt to harm her. Several scenes suggest that Kōichi develops deeper feelings for Mei, such as blushing in her presence and daydreaming about her. By the series’ conclusion, the two are shown walking home together, highlighting the strength of their bond.

Naoya Teshigawara
Kōichi quickly befriends Naoya Teshigawara, one of the first classmates to welcome him to Class 3-3. Their friendship is marked by casual banter, with Naoya giving Kōichi the nickname “Sakaki.” Although Naoya is obligated to treat Kōichi as “non-existent” after the countermeasure is applied, he later apologizes for the necessity. Kōichi often relies on Naoya for support due to his easygoing nature and relative independence from the class consensus.

Yūya Mochizuki
Yūya is one of Kōichi’s earliest allies, initially welcoming him to the class and later encouraging him, through a discreet note, to seek answers from Mei about the class curse. Kōichi respects Yūya’s artistic talents and collaborates with him during group activities, including the beach outing. Their friendship is portrayed positively in both the anime and manga, though the manga includes additional nuances, such as Kōichi occasionally expressing frustration with Yūya or commenting on his delicate appearance.

Reiko Mikami
Kōichi maintains a close relationship with his aunt, Reiko Mikami, who serves as both his guardian in Yomiyama and the assistant homeroom teacher of Class 3-3. She provides him with advice and guidance as he adjusts to life in the town. Their bond is portrayed as familial and affectionate, with Kōichi considering her a maternal figure in the absence of his late mother. In the anime’s climax, he is devastated to learn that Reiko is the “Extra” responsible for sustaining the curse. Although he resolves to kill her himself, he does so with visible grief, saying “Goodbye, mom” before the act. In the manga, their bond is emphasized further, with Kōichi referring to her as “Reiko-nee” and reacting even more strongly to her death.

The given name Kōichi (恒一) is composed of kō (恒), meaning “constancy” or “always,” and ichi (一), meaning “one.” Together, the name can be interpreted as “everlasting one” or “constant one,” emphasizing stability or continuity.

The surname Sakakibara (榊原) combines sakaki (榊), referring to the sacred evergreen tree often used in Shinto rituals, with bara (原), meaning “meadow” or “field.” The name thus conveys “field of the sakaki tree,” linking it to traditional Japanese cultural and religious symbolism.