""I wonder if we too will forget this. The trip, what’s happened since April, and everything that had to do with Reiko. Everything…""

- Kōichi Sakakibara

Kōichi Sakakibara (榊原 恒一, Sakakibara Kōichi) is the male protagonist of Another. He is a new transfer student to Yomiyama North Middle School's 9th Grade Class 3 in May 1998. He is the only son in the Sakakibara family. His mother supposedly passed away 15 years ago after giving birth to him. He also makes a minor appearance in phone calls during the events of Another 2001, where he is 18 years old.

Kōichi is a 15-year-old boy who is a transfer student from Tokyo. In Yomiyama, he lives with his grandparents and his aunt, Reiko Mikami, as his father works in India. He no longer has a mother, as she died shortly after giving birth to Kōichi. Kōichi has a great curiosity about the events 26 years ago that made the class so beset with the fear of death.

In the anime, Kōichi has a younger face, short brown hair, and brown eyes. In the manga, he has black hair and black eyes and looks slightly older.

He wears Yomiyama Middle School's uniform most of the time. For his casual clothes, he always wears two layers of clothing, consisting of a long-sleeve shirt and a T-shirt.

Kōichi is shown frequently to have a very curious personality, being determined to find out what happened 26 years before the main storyline, despite repeated warnings from Izumi Akazawa and the other students of Class 3-3 to not go "looking for things that don't exist". He also initially believes Mei Misaki to be a ghost due to nobody acknowledging her presence. Ultimately, his curiosity proves to be his undoing when the students decide to treat him like he doesn't exist as well. His feelings about this are ultimately mixed; while he doesn't particularly like "not existing", it is also a weight off his chest, as he now knows for sure that Mei exists, and the two bond over their mutual situation.

Generally, Kōichi is very calm, friendly, and sociable, making new friends since his first day at Yomiyama North Middle School and getting along fairly well with everyone. However, he can also be a bit dense, being unaware of the feelings some girls have for him, like Izumi and Yukari Sakuragi, or not responding to conversations in ways people want, like him describing Izumi in her perspective as "macho". He can also be rather naïve, as he can't understand why Mei and her mother (actually her aunt) treat one another like they are strangers, almost as though he can't comprehend family members not getting along with each other.

In his spare time, Kōichi enjoys reading, which is what causes him to form a friendship with the nurse Sanae Mizuno. He seems to favor American horror, including Stephen King, Howard Philip Lovecraft, and John Saul. Because of this, Sanae nicknames him Mr. Horror Lover. He has also been given other nicknames, such as Sakaki by Naoya Teshigawara and Kō-chan by Aya Ayano. Kōichi also mentioned wanting to go to a fine arts school to study carving and sculpting in a conversation with his aunt, Reiko.

Although sociable, Kōichi is also rather timid for the most part (in the novel, this is revealed to be the result of his being bullied in his previous school, due to sharing his last name with a serial killer) and easily horrified by the actions of the Curse. Over time, however, he gains a significant resolve to put an end to the Curse and protect Mei; the latter is proven when he willingly stands against the entirety of Class 3-3 in order to protect her, due to them believing she is the Extra. Even after learning that Reiko is the Extra, he insists on killing her himself instead of letting Mei do it, albeit not without great hesitation and remorse.

Kōichi also has a rather cold side to his personality; when Izumi ends up fatally wounded by the Curse and asks him if he remembers the first time they met, he tells her honestly that he doesn't, instead of lying to let her die happy, though the fact that she had just tried to kill Mei likely influenced this to some degree. The sheer bluntness of this statement is noted by Izumi as she dies, who tells Kōichi that he should have at least pretended.

Kōichi is first seen in the hospital recovering from a collapsed lung. He is visited by Yukari Sakuragi, Izumi Akazawa, and Tomohiko Kazami, each of whom introduces themselves to Kōichi. They begin to ask him strange questions, like if he ever lived in Yomiyama, something he says he hasn't done.

Izumi shakes his hand, and Episode 12 shows that Izumi met Kōichi when she hit him with a can due to her anger over her brother's death. Kōichi helps her get up. At the hospital, he also meets Mei Misaki in the hospital's elevator, and she tells him that her poor 'other self' is waiting for her in the morgue. When he is discharged, he finally gets to go to school and meets the rest of the class. Later at P.E, Kōichi first meets Ikuo Takabayashi, whom he tells to stay strong and not to let his heart conditions hold him back, and that he'll be able to run again. After he asks Yukari where Mei was, she acts as if she doesn't know what he's talking about, but he sees her on top of the roof and runs up to meet her. He asks her some questions, but she rejects them, saying that she hates being questioned. She turns around and tells him goodbye. In later episodes, Kōichi asks around the class for info about the class' curse, but no one was willing to tell him.

In Episode 3, he witnesses the death of Yukari after seeing her being quite shocked by his presence, causing her to fall down the stairs and her throat being impaled by an umbrella.

In Episode 4, he talks to Sanae Mizuno on the phone, she tells him about what happened the moment she asked her brother about the curse, but while they are still conversing, he hears the sound of the elevator that Sanae got on crashing, resulting in the nurse's death.In Episode 5, Kōichi insists on knowing about the curse, and so, while walking on the way home with Yūya Mochizuki and Ikuo, he asks again about Mei, and when Ikuo was about to answer, he had a heart attack and died later on. This incident caused Kōichi to be completely ignored as if he didn't exist. As he tries to understand what's going on and receives no answer until Yūya leaves the class roster in his desk, leaving a note telling him to let Mei explain everything. This leads him to visit Misaki M. Studio (Mei's house), and she explains why they were being ignored by the rest of the class. Their relationship develops after that, even leading Kōichi to fantasize about dancing in front of the class with Mei. Later on, they go to the library to look at the class yearbook, when they encounter Tatsuji Chibiki, who tells them more about the class' curse. At home, Kōichi tries to ask his family for info on the curse, but ends up empty-handed. The next day, their homeroom teacher comes to school with a knife and ends up stabbing himself in front of everyone. Kōichi is shown to get a small stream of blood down his face and was among the last of the students to leave the classroom. After the incident, he and Mei were back to being existent again. Later on, Naoya tells the others to meet up at the restaurant to discuss something. There, they meet Yūya's sister, Tomoka Inose, who tells them that it had a man named Katsumi Matsunaga, who stopped the calamity before; however, he can't remember anything. Afterwards, Kōichi tells Mei about what they found out, and Mei tells him that he shouldn't worry, because he isn't the one who is dead.They go to the beach to find Matsunaga and decide to relax until he returns. He meets Mei, and they talk about her family, and their hands touch. After, they go for different ingredients to eat. Mei asks Kōichi about the different seafoods she picks up, most of which aren't edible. She then finds an octopus, which latches onto her, and she tells Kōichi to take it off, afraid that it might eat her. After, they meet with Matsunaga, who tells them that though he can't remember much, he thinks someone died on the mountain when they went to the shrine. Suddenly, a strong wind blew the ball they were playing volleyball with out into the ocean. Junta Nakao says that he'll go and get it, but he goes limp. Kōichi and Katsumi try to save him, but can't because a speedboat comes out from behind the mountain. Kōichi watches in utter horror as the boat hits Junta, cutting him up and sending him flying. Kōichi, still horrified, looks at Katsumi, who remembers where he hid it, in the classroom. Kōichi went on the class trip. He, along with Naoya and Yūya, took Mei's side when Izumi blamed her for the deaths. Mei invites him into her room to show her the graduation photo of 1972, which faintly shows Misaki Yomiyama. Kōichi said that he found that photo in a room Reiko Mikami usually used. After Kōichi recognized the dead guy, Mei tells him about her cousin who is actually her twin sister, Misaki Fujioka, and all the story behind this and her doll's eye, that can see the color of the death in all the deceased people, Kōichi asks Mei who is the dead person, and when Mei was about to say the name, Naoya suddenly opens the door with a terrified look, saying that he had done something terrible.In Episode 11, Kōichi and Mei learned from Naoya that he had accidentally pushed Tomohiko off the building during an argument, and they went to look for his body. When they reached the hall, Kōichi noticed the dining room door is open, and he went to check. He found Manabu Maejima when he grabbed onto his leg. Manabu was literally stabbed in the back, and he told Kōichi that the dining room is on fire and the manager is inside. Kōichi went to open the door and found that the room is on fire, and the manager was sitting dead in the room. Kōichi went to assist Manabu and look for help when Izumi came to check on the ruckus. They later heard Takako Sugiura scream, and they went to check on it when they found the room is stained with blood and Takako is missing. Afterward, Takako appeared from behind, holding a blade, and tried to kill Mei. Kōichi defended Mei and brushed Takako off by removing her weapon. Takako escaped, and Izumi told them that they had listened to the tape, and Takako thinks that Mei is the dead person. After a second, Takako broadcast the tape and told everyone that Mei is the dead person and to send the "dead back to death." Many students came out of their rooms holding weapons in an attempt to kill Mei. Mikami-Sensei rushes over to the scene in order to defend Mei. Mikami is accidentally knocked out by Yukito Tsujii, and Kōichi and Mei make a run for it. Kōichi protects Mei, but is later caught by surprise when Takako stabs him in the arm. She swings at Mei, but misses, and gets tangled in a few metal cords. The ceiling falls, causing the cords to go upward, choking Takako to death. Izumi arrives shortly after and finds Kōichi and Mei standing underneath Takako's dead body. Izumi becomes horrified and enraged, thinking Mei killed her. Izumi vows to kill Mei. Mei runs up the stairs, and the ceiling falls behind her, blocking the path. Kōichi is left behind, but is still able to move. In Episode 12, Kazami tries to kill him, but Izumi arrives in time to stop that. Later, Kōichi protects Mei from Izumi, who tries to kill her. When Izumi finally gets a hold of Mei and Kōichi, she attempts to kill them both, but at that moment, lightning strikes. The glass shards are sent flying towards them. Izumi ends up being skewered against the wall by the glass shards. Kōichi is shocked and removes her from the wall. While lying on the floor bleeding, Izumi tells Kōichi that they had actually met before, one and a half years ago. Kōichi doesn't remember and says that he doesn't. Izumi smirks and tells him that he should at least pretend to remember, and she dies. Mei then disappears. Kōichi called her and asked her where she is; she told him, and also said that he doesn't want to come there. Kōichi realized Mei is trying to kill the dead one and goes there only to find out that his aunt, Reiko, as Mikami-Sensei, is the dead one. At first, he didn't believe it, but Mei comes with strong reasons, claiming that she witnessed Reiko's murder, and Kōichi comes to the realization that Reiko is actually dead and offers himself to kill her. Before killing her, his memory flashes back to when he visited Yomiyama a year and a half ago, the main purpose of his visit was Reiko's funeral (his memory having been altered by the curse). They are later seen at Reiko's grave, talking about the incident with Chibiki. Afterward, Kōichi and Mei then walk together, talking about what happened and that only they can see Reiko in the photo taken on the class trip. And also telling Mei that he tried to call her when he was in the hospital, but he couldn't connect, with her saying that she threw her phone because she hates those devices, but would probably get a new one. He then asks her if he could sometimes call her by the time she gets a new one, and she said, "Sure, sometimes."

Then they talk about how they're the only ones to see Reiko in the class trip picture and claim that they're the only ones to remember her, but Kōichi suddenly became sad about the fact that maybe later, they would forget everything that had happened. And he later asks: "It's over now, isn't it?", but Mei only replies with a smile.

He feels very strange about her because of the class treating her as though she "doesn't exist". He met her in the elevator in the hospital during his last day hospitalized. When he finally enters school, he sees Mei sitting alone with no one to talk with, and her desk is far older than the others. Every time he sees her, he goes to her place and asks questions. Even though Mei and his other classmates warned him to stay away from Mei Misaki, he still hesitates and decides to visit her every time he sees her. He also seems to be very concerned about her. In chapter 10, he slightly blushed when he saw Mei, which confuses her. In Episode 6, Kōichi daydreamed about dancing in the classroom with Mei, and seemed embarrassed when he snapped out of the daydream, even telling himself that he had to stop doing that, perhaps indicating that he is developing some kind of feelings for her. At the beach in Episode 8, he is the first one to notice Mei is there, and he goes over to her and helps her rebuild what seems to be a sandcastle, and while doing so, their hands touch within the mound of sand. It is this scene that seems to show that they have a good friendship. He also helped her when her hand was covered by an octopus. In Episode 10, he also stood up for Mei when Izumi told Mei to apologize for all the bad luck and mishaps that had happened to their class. In Episode 11, Mei was accused of being the dead one by Takako. In the same episode, right after Yukito hit Ms. Mikami in the head with a broom, knocking her out, and Kōichi punched him as a result, he was just about to go up against Kenzō before Mei showed him not to go through with it. As he and she were being hunted down by the others, he and Mei agreed to hold hands, showing she trusted him enough and that he wanted to make sure she wouldn't get killed. Kōichi protected Mei and helped both of them escape the situation, as Takako, along with their classmates, tried to kill Mei. At the end of the series, he and Mei are seen walking home together.

Kōichi and Naoya are on good terms. Naoya was one of the first to talk with Kōichi on his first day at Yomiyama Middle School, being part of the group that directly welcomed him. Later on, Teshigawara apologizes for the fact that he is obligated to ignore him when he became a non-existent person. Kōichi thinks that Naoya's casual, flowerish T-shirt is too attractive and embarrassing. Evidence of them being on good terms, too, is when Naoya nicknames and calls Kōichi "Sakaki". Due to Naoya's laid-back attitude and not going with the rest of Class 3-3 most of the time, he's the person Kōichi would ask to help him in solving the curse.

Yūya was the closest person to Kōichi early in the series, when he was a newcomer at Yomiyama North Middle School. He was among the group of students who welcomed Kōichi directly to Class 3. Also, Yūya was the one who left Kōichi a message in which he says to talk with Mei about the class mystery, as he was obligated to keep quiet about this. They are on good terms. Kōichi and Yūya were also on the same team when they were at the beach with Mei. Kōichi is also one who is interested in Yūya's art talents and says he's talented in it. In the manga, they are still friends, but Kōichi's thoughts on Yūya were a little judgmental, like him taking some anger out on him, and one to state his slight feminine appearance, stating that Yūya would most likely get picked up on the streets by accident.

Kōichi is very close to his aunt, Reiko. She gives Kōichi lots of advice on how to get by living in Yomiyama. Also, Kōichi is often seen talking to her. In Episode 12, Kōichi was shocked when he finds out Reiko is the extra one. Before killing her, Kōichi said, "Goodbye, mom". This fact claims that their relationship was very close, and Kōichi considers her the mother he never had. In the manga, Kōichi often refers to Reiko as "Reiko-nee" (as in nee-san, Japanese for "big sister"); he was more affected by her death as he was seen crying. In the manga, they seem to have a stronger bond, and often she makes him blush.

In episode 12, it is hinted at why Kōichi does not remember going to Yomiyama a year and a half ago. The reason for his coming is Reiko's funeral, and thus, his memory of going there and meeting Izumi Akazawa is altered. It is also revealed that he knew about Reiko's double life as his aunt and assistant homeroom teacher to his class. Reiko told him this when she was stating the class rules to him, and the last one was to refer to her as Ms. Mikami, never Reiko. This also explains why Kōichi was embarrassed and unwilling to go with the others' discussion about Reiko being hot, and Yūya's asking him about her was very minimal.

Yukari first met Kōichi at the hospital in Episode 1. She gave him a bundle of flowers that the whole class put together. She was among the students who directly welcomed Kōichi to Class 3. She also sat with Kōichi during P.E. class when he was recovering and she had a sprained ankle. Kōichi probably saw her as a good friend, oblivious to the fact that Yukari flirts with him. The two also share a bonding scene where they walk home together and discuss the class trips. She was also a main person who started giving Kōichi clues in the past in Class 3. He is very guilty over her death, showing he cared for her as well as everyone else.

Kōichi first met Tomohiko at the hospital with Izumi and Yukari in Episode 1 of the anime. As the group spoke, he gave Kōichi notes from the school year so far to help him. He was among the students who welcomed him to Class 3 directly, and the two seemed to be on good terms with each other, with Tomohiko even calling Kōichi by his first name, either because they were real friends or he did it because Izumi did. All that changed, however, at the inn when Tomohiko started to kill their classmates, and then, when they encountered each other, he attacked Kōichi. Despite speaking the truth, it's shown that Kōichi mostly defends rather than fights, as not want to hurt Tomohiko. After Tomohiko screams at Kōichi for causing Yukari's death, Kōichi says her name while telling Tomohiko to stop. Right before Tomohiko strikes Kōichi with him pinned on the floor, he's hit in the head by Izumi, with Kōichi startled and letting her rant on Tomohiko's foolish thoughts and her helping him die for his actions. After Izumi runs off after an encounter and discussion with Chibiki, both bring Tomohiko's dead body outside, where it's shown that Kōichi is among those who were saddened by his death.

Kōichi was shown to be on good terms with Ikuo during the series, as shown when Ikuo called Kōichi by his first name. Ikuo was shown to be among the students who welcomed Kōichi to Class 3. The two talk to each other during P.E. when both were sitting out, Kōichi learned about Ikuo's heart condition and knows he can't run, with Kōichi trying to convince Ikuo that he might be able to run one day with true sympathy. As Ikuo suddenly felt ill, Kōichi offered to help him to the nurse's office, but he states he could get there. Later on, after a whole class meeting without Kōichi, he is about to head home with Yūya, when Kōichi asks to walk home with them in order to try and get more information on the past. While walking and talking, Ikuo decided to try to help Kōichi despite Yūya's concerns, as Ikuo disliked the ways Izumi and the others were handling the situation. Right after Kōichi asks about Mei, Ikuo suffered a heart attack while Kōichi and Yūya were in shock. Later on, it's shown that Kōichi felt guilty over his death, as seen when Ikuo appeared in his nightmares.

Kōichi was shown to be on good terms with Aya, as seen in the series. She was among the students who directly welcomed Kōichi to Class 3 in Episode 1. She also questioned his taste in women and called him by his first name, showing she considered him a close friend (In the Japanese version, she calls him Kō-chan with the "chan" meaning endearment and reserved for close friends). In Episode 4, the two talked after Kōichi came from the hospital and Aya was skipping class. While they talked, a gust of wind blew a window pane off a nearby truck, and Kōichi thought quickly and saved himself and Aya. After Aya screamed that she didn't want to die, this was helpful in Kōichi's learning and questioning about the curse and current events. Later on in Episode 9, while Kōichi and Teshigawara were going to the old school building to meet Yūya and search the old Class 3 for Matsunaga's clue, Aya spotted them along with Yumi. The girls discuss their looking for Chibiki, who's revealed to be the drama club teacher, much to Kōichi and Teshigawara's shock. She asked Kōichi directly why the boys are here, and he explained to them, with both hopeful in them, looking for a solution. After denying Teshigawara's offer to help them, Aya's reason was that she was moving that day. She says goodbye to Kōichi, this being the last time he'd ever see her. It's probable that Kōichi found out about Aya's death and was saddened by it off-screen.

Kōichi is shown to have respect for Chibiki for his role in the school. Chibiki is shown to be very useful in providing Kōichi with information on the past with the Curse and the Class, especially since he was its homeroom teacher. He's shown to have known Kōichi's mother and aunt when they were students, and he was a teacher back in the past. He is known by Reiko as dressing in black and as the Master of the library when talking to Kōichi. He is also the one to explain the methods behind the deaths of Shoji Kubodera and Junta Nakako to Kōichi and Mei. He also is shown to care about the safety of the students to the best of his abilities, as he saved Kōichi, Naoya, Yūya, Tomohiko, and Daisuke.

Kōichi's interactions with the rest of his classmates are very limited to none whatsoever, except with a few who interacted with him fully, like Makoto, Noboru, Takako, Junta, and Yumi. In the anime, Kōichi had a few interactions with them, but due to the curse and the new rule put out, they all ignored Kōichi together. At the inn, Kōichi has the longest interactions with them. Kōichi helped Manabu when he was stabbed and wounded, confronted Yukito after he hit Ms. Mikami, went through multiple groups to get Mei to safety when she was going to be killed by them, tried to help Aki when she was chased by Tomohiko, and was with the small group at the hospital to visit Manabu in the series finale.

Kōichi freely admits to Reiko that Yōsuke isn't a very good dad, and is often frustrated with his apparent lack of parental interest, among other things, having learned to cook as a result of his father's frequent absences.

In the anime, Yōsuke seems to be a bit better of a father, explaining he'd never mentioned his own lung problems to Kōichi for fear of someone telling him it was a hereditary problem, suggesting he didn't want Kōichi to develop a complex over it. Throughout the series, Kōichi is shown to be uninterested in his father explaining India's hot weather. Kōichi also gets some facts about his mother in school from his father, and even asks once if there were any pictures he could look at for clues.

Yōsuke is only indirectly mentioned in the live-action film.

They were quite close, having met while Kōichi was in the hospital. The two met outside the hospital a few times, like at a restaurant and the Hospital lounge, and Sanae had his phone number, which she used to help him research the Curse. She often called him nicknames, such as "Mr. Horror Lover", as she would recommend certain books for him, like Stephen King and H.P. Lovecraft, and wanted to know about the mystery of class 3. While interacting, it is shown she thinks of Kōichi as much as a younger sibling as her own, as she seems happy that Kōichi is more focused on reading rather than her brother is. While talking at the restaurant, she teases him at one point when she suspects Kōichi may have had feelings for Mei Misaki, although Kōichi tries desperately to get her not to think that. San was probably the best person who gave Kōichi information on the curse, like finding the file of Mei's Sister, Misaki, before her death. Her death devastated him, especially in the manga, as he can be seen crying.

Their relationship differs very differently from the manga. In the anime, they appear to be somewhat close and, in some way, on good terms, as with Yukari, Kōichi didn't fully see that Izumi had feelings for him, as throughout the series, he was very preoccupied with Mei. It is shown that he might have better understood after Takako spared him, as she said Izumi was fond of him. As Izumi fought, he tried to stop her but not harm her, and at first, it looked like she understood him when they stared into each other's eyes, but she kicked him back and threatened him and Mei. As she died, she made amends and called him terrible, saying that even if he didn't remember meeting her, he ought to pretend. It is revealed that she was fond of him. She remembers Kōichi being in Yomiyama a year and a half ago because this memory has not been altered. This is because even though this memory is connected to Reiko Mikami's death, her name is not mentioned. The reason Kōichi does not remember his meeting with Izumi is that he knew before that the reason he was in Yomiyama before was because of Reiko's death (this memory was altered by the phenomenon, making him forget this encounter).