Kazuki Saitoh (斉藤和樹, Saitō Kazuki) is a recent university graduate employed by Murder, Inc. He is transferred to the Fukuoka branch after failing an assassination assignment, and after leaving the killer industry, he resumes a mostly normal, quiet life. He is a member of the Hakata Tonkotsu Ramens baseball team.
Saitoh has short brown hair, brown eyes, and is usually seen wearing a brown or green business suit with a red tie in illustrations.
He is often described as timid and fearful and is prone to tearing up under stressful situations. While he had been taught the necessary techniques of hand-to-hand combat and knife wielding at Murder, Inc., he has never made use of his training and has forgotten nearly all of it. To make up for his own inabilities, he often depends on others to help him resolve issues beyond his capabilities, such as being framed for murder or targeted by assassins.
Saitoh has a huge interest in and love for baseball. In his thoughts, he frequently compares current situations he is facing to particular set-ups that occur in baseball games. Because of a past incident where he accidentally hit a batter in the head with a pitch, he had developed the 'yips' and become unable to throw a ball, afraid he would end up hitting and injuring someone again. Although still timid and easily frightened, he overcomes this with the help of his new friends and is able to pitch again after the Mayor Harada scandal.
Saitoh was part of a veteran school in the Kanto region of Japan and participated in their baseball club, which he was the ace pitcher of the team. His school team managed to enter Hanshin Koshien, where the Japanese National High School Baseball Championship is held. During one of the matches, however, Saitoh pitched a ball that hit the batter in the head, which ended with the batter hospitalized and in a coma. Saitoh was traumatized by the incident and, unable to pitch, resigned from the team and focused solely on his academic career.
Mayor Harada Scandal Arc
Saitoh is accepted to an interview for a job position at a corporation in Tokyo, completely unaware he is applying to an assassination company, Murder, Inc. He manages to impress one of the interviewees by reciting the time he almost killed someone at Koshien in the past. He receives an offer two weeks after the interview. When he arrives at the company he is surprised by their true operations. After months of undergoing training he is given his first assignment to assassinate a political figure but can't muster the courage to do so. Because of his failure, he is directed by his superior to transfer to the Fukuoka branch the following day or risk being eliminated by the organization.
Once he arrives in Fukuoka, he gets situated and heads to meet his new superior, who gives him a fake ID and an assignment to kill Jun Murase. After receiving the details of his target, Saitoh heads to Murase's residence and waits for him to return home to complete the task. He is visited by Jiro and Misaki shortly after, and unaware that they also have come to capture Murase, he gives his target's name instead of his own, resulting in his kidnapping.
He wakes up in an empty warehouse, tied and gagged, and finds himself about to be tortured by Jiro and Martínez for the crimes committed by Murase. They first avenge the cats tormented and killed by Shota Yamazaki. Saitoh is forced to watch hours of torture and the boy's decapitation. Jiro prepares to beat Saitoh up under the assumption that he is Murase, but when the tape falls off of Saitoh's mouth, he takes the opportunity to explain himself and prove his innocence by showing him his business card from Murder, Inc.
After Saitoh is let go, he returns to his hotel and rests until he is woken up by a phone call from his superior, congratulating him for killing Murase. Saitoh immediately calls Jiro to ask if the avengers had killed Murase but is informed they did not. While wary of the circumstances of Murase's death, Saitoh nonetheless appreciates getting to live another day. With the huge amount of money he receives for the job, he decides to have fun in Nakasu. After partying at various clubs he meets Reiko Asakura and is led, very drunk and drugged by Reiko, to a hotel.
The next morning, he wakes up in the hotel with little clothing and finds a naked woman in bed next to him, dead. Unable to remember everything from the night before and realizing the dangers of staying in the same room as the body, Saitoh flees in panic. While wandering through the city he sees a television broadcast announcing the discovery of the woman's body and his own face, wanted as a murder suspect. Desperate for help, he hurries over to Bar Babylon, seeking assistance and shelter from Jiro. He explains the details to him, and Jiro suggests he was likely framed. Wanting his name cleared, Saitoh tasks Jiro to avenge him by placing the blame back on the real culprit.
Saitoh stays hidden at the bar while Jiro is out making preparations. When Jiro comes back, he tells Saitoh that he discovered who set Saitoh up and takes him to an apartment building in the city. They meet up with Xianming Lin and Zenji Banba. As the Niwaka Samurai, Banba calls Reiko, and she attempts to flee the apartment. Saitoh confirms that Reiko is the one who he had encountered in Nakasu while drunk, and Jiro captures her as she tries to escape. Saitoh is left in the apartment entryway while Banba and Lin head inside. Just as Yusuke Harada ambushes them with a gun, prepared to shoot, Saitoh quickly grabs a signed baseball on display next to the door and pitches it at Yusuke's back, incapacitating him. Impressed by his technique, Banba and Jiro ask Saitoh to join the grasslot baseball team they belong to, the Hakata Tonkotsu Ramens. Saitoh promises to consider their offer.
Afterward, Jiro takes him to a nearby hotel and gives him Saeki's business card, suggesting that Saitoh change his facial features to escape both the murder charges and Murder, Inc. After the operation, deciding that his new face provides him with a new chance at life, he calls Jiro and accepts the pitcher position. He attends his first game with the team the following week, during which he is struck on the head with a pitch while in the batter position. He realizes that the pitcher looks similar to the batter he had hit with a ball years before and that Jiro and Misaki are smiling.
Niwaka Samurai Impersonator Arc
While Saitoh is rediscovering his love of baseball, being hit on the head has given him a fear of high pitches, and he unconsciously dodges when a ball is pitches too close to his head. Other teams are quick to note this and use it to their advantage. During a practice game in late June, he is worn out by pitching in the heat during the team's long, poor defense. Exempted from the punishment game and exhausted from pitching, he strikes out at bat, yet continues cheering on his team, noting how seriously his teammates take the game and how hard they work to win.
Saitoh quits Murder, Inc. and begins working as a sales clerk at a convenience store, successfully hiding from the corporation a half year. On one of his shifts, he is confronted by Yamamoto, who threatens him for all the money in the cash register and some food off the shelves. Saitoh follows his orders until a mysterious customer in a trenchcoat disarms the robber. After Yamamoto flees, Saitoh formally meets the regular, George Gondo, and gives him his favorite coffee in thanks.
A few days later, Saitoh is returning home after one of his shifts when he receives a voicemail from Enokida, warning him of an assassin from Murder Inc. who may be at his house. He and Nguyen spot each other at the same time. Saitoh begins running, desperately trying to get away, and calls Banba for help, only for his roommate Lin to answer instead. Lin hurries to Hakata Station and gets on the same train as Saitoh. Before Nguyen completely corners him, Saitoh is rescued by Lin. He hides and watches their fight, and once Lin immobilizes Nguyen, he goes to Shigematsu's place to hide out. With a killer after him, Saitoh decides to quit his job, but he agrees to work one final shift.
At the shop, Saitoh is approached by Gondo, and before he can thank the man again, Gondo attacks him with a blade. Shocked, Saitoh dodges his swings and runs, frantically calling his friends, who either cannot answer their phones or turn him down. He spots Enokida crossing a street as he runs through Nakasu, and Enokida runs away from him. With Saitoh in pursuit and a killer behind him, the pair run into Genzo, who incapacitates Gondo with one punch. After hearing his story, Saitoh sympathizes with the man. Enokida instructs Gondo to tell his client that a killer from Murder, Inc. beat him to the kill, and he reassures Saitoh that he can find a way for the corporation to stop sending assassins after him. The recently-captured Yamamoto is used as a stand-in, going through cosmetic surgery to look like Saitoh before being handed over to Nguyen. With his death official from both Murder, Inc. and Reiko Asakura's perspectives, his life is no longer targeted.
Shou Wang Arc
In the first week of September, Saitoh and Shigematsu head to the fireworks display in the eastern ward before the rest of the team in order to save a spot for their group, and the Tonkotsu Nine enjoy the fireworks show together.
Since quitting his job as a convenience store clerk, Saitoh has been looking for work. When he receives an email response from a corporation he had not applied to, he takes his chances and downloads the attached file, infecting his computer with a virus. Death and bomb threats sent to government officials are traced back to his computer, but without any knowledge of the threats or the platforms they were posted on, he voluntarily submits himself to questioning with cybercrime investigators. While in questioning, his phone begins sending photos of naked middle school-aged girls, proving that it is being remotely controlled. With Enokida vouching for him, he is declared innocent and released.
He is still shaken up by the incident hours later, and he visits Gen-chan, where he and Genzo explain the situation to Martínez and Genzo treats him to a sympathy meal.
The following day, Saitoh finally lands a normal, salaried job, and Genzo treats him to a congratulatory meal. As he heads home early, intending to get ready for his first day of work the following day, a thug is waiting for him outside of his apartment, pursuing the bounty that Enokida has placed on his head. When Enokida does not answer his calls, Saitoh frantically calls Lin, who tells him to spend the night in Shigematsu's apartment and hangs up on him.
A few hours before sunrise, Enokida slips out of Shigematsu's apartment, intending to go home and prepare for his first day of work. The same man is waiting outside of his apartment. As Saitoh runs off and corners himself in a dead-end alley, cursing the informant, Yagi appears and knocks the thug unconscious. While Saitoh is apprehensive of another potential hitman, the butler thanks him for being friends with Enokida and presents him with a large sum of cash and souvenirs from his vacation.
At one of the Ramens' weekly baseball practices in early fall, Banba asks Saitoh to pitch some sinkers for him to practice hitting, an odd request that ends up being useful a few days later during Banba's fight with the Submarine Ninja, Shunsuke Saruwatari.
Saitoh pitches for the Ramens' pennant race practice game, managing to keep the team in the lead for most of the game until Lin makes a blunder in the eighth inning. At the end of the game, he joins in celebrating Martínez' home run and the team's victory.
- Saitoh's given name, shown on his business card, was first revealed in the anime and later in volume nine.
- The convenience store he works at in the second light novel is a Family Mart near Canal City.