Yamamoto is a young man with an aged face caused by his drug addiction. He has a poor complexion and dark bags under his eyes. The cannabis he frequently smokes makes him speak in an unintelligent-sounding drawl on top of his lisp. He wears overalls with a white towel tied around his head, making him appear to be an ordinary, blue-collar worker. He sometimes wears a ski mask while killing.
He is exceedingly lazy and is unable to work alone. Being self-centered, Yamamoto neither notices nor cares how his laziness and carelessness drag his partner and others down with him. Recognizing Abe as his caretaker of sorts, he shows concern for his partner, knowing that he can not function independently. He is addicted to cannabis and claims that he is unable to work without it, as it prevents his hands from shaking. The cannabis causes hallucinations, however, and his addiction, carelessness, and stupidity severely inhibit his memory.
Yamamoto used to make a living as a convenience store burglar and purse snatcher. Six months prior to the start of the story, when the numbers of teams of killers in Fukuoka began to increase, Abe decided to look for a partner and posted a notice on Undergroundjobs.com. Yamamoto responded to his notice, and despite Abe's wariness, he was accepted as Abe's partner.
During their first job together, Yamamoto's hand was shaking from the drugs, and he nearly shot Abe. On their second job, Yamamoto carried two guns for the visual effect, and the ejected cartridge from one hit his other hand and bounced into his face, and his second shot nearly hit Abe.
Niwaka Samurai Impersonator ArcEdit
Inside the pair's van, Yamamoto smokes pot while scouting a location for their third job. He incorrectly counts the number of targets that enter the building he is watching, causing Abe to be surprised and shot by one of the men inside. Yamamoto shows concern for his partner and calls him by his real name in front of their surviving victims. Due to his injured hand, Abe allows Yamamoto to drive, but his paranoid reaction to another driver while stopped at a traffic light leads him to drive between two lines of traffic, hitting countless cars stopped on Watanabe Street.
Yamamoto and Abe visit their mediator, Genzo, and overhear him talking about the Niwaka Samurai. Genzo tasks them with assassinating another one of his killers, a man who had killed the wrong target and is blackmailing Genzo for consolation money. When the man visits and departs Gen-chan, they tail him until they lose him in traffic. They up split to relocate him in Nakasu, and Yamamoto spots the man in the blue shirt exiting a convenience store. He calls Abe to report that he has found the target and comment on the similarity between their current assignment and the work of the Niwaka Samurai. Yamamoto follows him to a parking lot, and, worried that the man will drive away and they will lose him, hurriedly kills him. When Abe catches up, he notices that the victim's blue shirt is different. He finds identification for Tadafumi Izuku in the man's pocket and realizes that Yamamoto has killed the wrong person.
They stuff the body into their van, and Abe devises a plan to use Genzo's help in finding the target again. Yamamoto volunteers to talk to Genzo, but he reveals that they killed the wrong target, and their mediator cuts them off from his business. Thanks to Genzo's influence, other mediators in Fukuoka are aware of the pair's blunder and turn them away. Defeated, Abe informs Yamamoto that their partnership is over. Almost in tears, Yamamoto threatens to go to the police and decides to buy Abe some food in consolation.
Yamamoto dons his ski mask and enters the convenience store where Kazuki Saitoh works. Wielding a knife, he demands the money from the register as he shops for food and drink items. George Gondo, a regular customer, disarms him effortlessly, and he grabs a small amount of money and food and runs from the store in defeat. Saitoh chases him outside, throwing a paintball at his receding back that splatters and dyes his clothes orange. When he returns to the van some time later, he gives the spoils of his burglary to Abe. Reading the newspaper over his partner's shoulder, he becomes inspired by a news story concerning celebrity impersonation and fraud. Yamamoto brainstorms the idea of posing as the Niwaka Samurai online in order to meet with clients and flee with the advance pay. He posts an advertisement on Undergroundjobs.com and receives a reply within minutes.
With Yamamoto dressed as a traditional samurai, the pair meets with Kumiko Izuku, who offers them a 100,000 yen advance payment for the murder of Kaori, the woman with whom her husband has been having an affair. Due to their suspicious appearances, she follows them and takes photographs of the pair and their van without their knowledge. They disappear with the money and spend the next week making small profits through this scheme.
When Naoya Nitta discovers their posting, Shunsuke Saruwatari meets with the pair. Though having realized beforehand that the samurai is a fake, he holds Yamamoto hostage until Abe has made contact with the Niwaka Samurai's mediator and obtained a meeting for Saruwatari with the Niwaka Samurai. As the pair leave Canal City, Yamamoto finds George Gondo's online post for the Niwaka Samurai, dead or alive, and devises a plan to abduct the Niwaka Samurai from his meeting with Saruwatari and cash in the reward.
As Saruwatari is about to finish off the Niwaka Samurai, Abe and Yamamoto drive in and take him before Saruwatari can recover from the surprise. They bring him to a warehouse and tie him to a chair, where Abe notices that the man before them does not match the description they were given. Though Yamamoto punches him repeatedly, they are unable to extract information from him and turn to Undergroundjobs.com to hire a professional torturer.
José Martínez arrives with a suitcase of torture supplies, and Abe and Yamamoto leave the room as he begins to work. When they reenter, the corpse has been mutilated beyond recognition, and Martínez has discovered that the man was friends with the real Niwaka Samurai and had come in his place.
Saruwatari and Nitta find them shortly after, and Saruwatari punches them both in the face, causing Yamamoto's nose to bleed profusely. Nitta holds a gun on them, while Saruwatari searches the car for his money. He is furious when he finds the corpse, but after Abe and Yamamoto explain that the corpse was a fake, Saruwatari settles for the contact information of the samurai's mediator and keeps the body, letting the pair go.
Immediately after, Abe receives a call from Genzo, offering them work. Abe meets Genzo alone, not wanting Yamamoto to mess up like the time before. They meet their clients, a man and a young girl, in a park the following morning. Before the man, Tanaka, can provide them with details of the job, his daughter asks Abe to play. Agreeing to amuse her, Misaki decides to play doctor and pricks him with a paralyzing serum in a syringe. Yamamoto immediately jumps in the van and tries to flee, readily leaving Abe behind. Misaki suggests shooting him, but Jiro shuts the idea down. Before he can give chase, a large SUV crashes into the van, and the torturer emerges from the driver's side, greeting Tanaka and the girl. He claims that the avenger work for the Watanabe Street hit-and-run is complete, and as they congratulate each other on a job well done, Yamamoto climbs out of the badly dented van, mostly uninjured but bleeding from a head wound and calling for an ambulance. Yamato arrives, and, spotting Yamamoto, who has fallen unconscious, takes his wallet and reclaims the rest of Kumiko Izuku's money while his friends look on in disgust.
With Tadafumi Izuku's murder left to avenge, Yamamoto is given to Shinichi Saeki. When he awakens, he is on an operating table under strong lights and can see a doctor above him. When he next reawakens, he expects to be in a hospital bed, but he is on a warehouse floor with his wrists and ankles bound. A silver-haired man standing above him reminds him of his mistakenly killing an innocent man several weeks earlier and asks if he would not mind dying in someone else's place as well. As he is speaking, a foreign man enters the warehouse and thanks him for his help, greeting Yamamoto and calling him Saitou. He promises to hurt Yamamoto badly before killing him, and as the man picks him up and carries him away, Yamamoto sees in a reflection that his face and hair have been changed, but with his mouth heavily taped shut, he cannot argue.