Saitoh has brown and well-kept hair and is usually seen in a formal brown or green business suit in illustrations.
He is often described as timid and fearful and is prone to near tears under stressful situations. While he had been taught all the techniques and skills in hand-to-hand fighting or using knives at Murder Inc. he has nearly forgotten all of that knowledge. Due to his inabilities he will often thus depend solely on others to help him resolve issues such as handling assassins sent after him or an incident he was framed for.
Saitoh does have a huge interest and love for baseball. In his narration in the light novels he frequently makes comparisons of current situations he is facing to particular set-ups that occur in baseball games. Because of an incident that happened in a game where he accidentally hit the batter in the head with a pitch he made he developed the 'yips' and has become unable to throw a ball, afraid he would end of hitting and injuring someone the same way as in the past.
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 make it 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 received trauma from the incident and, unable to pitch, resigned from the team and continued on solely with his academic career.
Mayor Harada Scandal ArcEdit
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 of 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 arriving in Fukuoka City he gets situated and then heads to meet his new superior who gives him a fake ID with the name Takuya Itou and an assignment to kill Jun Murase. After receiving the details of his target, Saitoh heads to Murase's residence and waits for Murase to return home to complete the task. Before he can do that, he encounters Jiro and his helper Misaki and unaware that they also have come to capture Murase, he gives his target's name instead of his own when asked, resulting in him being kidnapped by the two.
He wakes up in an empty warehouse, tied up and with his mouth sealed with tape and finds himself to soon be tortured by Jiro and Martínez for the crimes done by Murase. They first proceed to torture the high school student Shota Yamazaki who was tortured the same way he tormented cats in front of him. After they finish off the boy, Jiro prepares to beat Saitoh up under the assumption that he is Murase. Once the tape is removed from Saitoh's mouth, he takes that 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 calls immediately afterwards to ask Jiro if the avengers killed Murase but is informed they did not. While weary how Murase had been killed, Saitoh nonetheless appreciates he got to live another day and with the huge amount of money he received for the job decides to use some of it to have fun in the entertainment district. After partying at various clubs and very drunk, he comes across a disguised Reiko Asakura and is taken to a hotel with her.
The next morning he wakes up at the hotel with little clothes on and finds a naked woman next to him in bed, 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 the broadcast announcing the discovery of the woman's body and he is wanted under his fake name Takeda Itou 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 likely was framed. Wanting vengeance, Saitoh tasks Jiro to avenge him by placing the blame back onto the real culprit.
Saitoh stays hidden at the bar while Jiro is out making preparations. When Jiro comes back, he tells Saitoh that he found out who set Saitoh up and takes him to an apartment building in the city, and meets up with Xianming Lin and Zenji Banba (in the Niwaka Samurai disguise). Banba calls Reiko to make her run out of the room. Saitoh confirms Reiko is the one who he encountered on the street, and Jiro captures Reiko as she attempts to escape. Saitoh is left outside the room while Banba and Lin prepare to head inside. Just as Yusuke ambushes them with a gun, prepared to shoot, Saitoh quickly grabs a signed baseball from the cupboard next to the door and throws it at Yusuke's back, incapacitating him. Impressed by his pitching technique, Banba and Jiro ask Saitoh to join the grass-lot baseball team they belong to, the Hakata Tonkotsu Ramens, as their pitcher, which Saitoh tells them he would think over it for a bit.
Afterwards, Jiro takes him to a nearby hotel and gives him the business card of Saeki's cosmetic surgery clinic and suggests to Saitoh to change his facial features. Saitoh sees this as an opportunity to escape both the murder charges and Murder Inc. and heads to the clinic some time later. After the operation, he calls Jiro and accepts the pitcher's position.
He attends his first game with the team, and during the game he's hit on the head with a baseball by the opposing pitcher while in the batter position. He realizes that the pitcher looks similar to the batter whom he hit with a ball years ago and that Jiro and Misaki are smiling, seemingly having set up the incident.
- Saitoh's Murder Inc. business card in the anime lists his first name as Kazuki.