|Anime debut||Episode 19|
|Manga debut||Chapter 50|
|English Voice||Josh Morrison|
Yamagishi is a tall, well built man, who has brown hair and brown eyes. In the anime, he has a scar on his forehead. He is usually seen with a formal suit and tie. During the operation against parasites, he wears combat gear.
Yamagishi is very confident in the skills of his task force and himself. He showed a dismissive contempt of parasites, calling them nothing more than "machines" and treating them as simple pests. He was smart enough to deduce that Gotou was stronger in closed spaces and ordered his squad to the roof. However, he also possessed something of a sadistic nature, such as killing a human who was refused to obey his order, and sacrificing his troops who were being used as shields by Gotou. Though he became distraught over the death of his teammates, he fought until the end.
Yamagishi was first seen heading towards a meeting along with Hirama, when he comments on the apparent promotion. During the meeting, Hirama suggests incorporating Uragami into their strategies. Yamagishi respectfully declines, believing a cold-blooded killer would leave too many uncertainties in their plans. Hirama reminds him that Uragami wouldn't be considered a member of the unit, but just an extra pair of eyes. Yamagishi states that he'll most likely be unnecessary due to the scanners created specifically for locating parasites. Continuing the discussion Hirama says that they won't be herded like sheep. Yamagishi says Hirama is thinking in a very "policeman" type of way and that they're not dealing with humans, but pest control.
Yamagishi is later seen discussing tacticians in a room with Hirama and other multiple people involved in the assault. They will use eight buses to block the main parking lot. They will quickly surround the building calling it their "outer wall".
Now inside the bus himself, wearing combat gear, he proceeds to begin a broadcast. The broadcast declares that a lone assailant, who is armed and dangerous, has taken refuge on the roof and that everyone in the City hall building must locate to the main floor. Yamagishi states that Japanese people become scared when the military becomes involved in most situations, making it more convenient to lie and proclaim themselves as a police task force. Yamagishi then sets up their scanners, saying the trickiest part is about to begin. As the civilians located on the main floor are evacuated, the scanner will begin X-raying their bodies revealing anything nonhuman in their anatomy. He watches on as the first few groups of seven leave the City Hall. As the first one is identified, he comments on the parasite's gender saying "ladies first".
In a flashback, Yamagishi enters a room where Shinichi and Hirama are seen waiting. Yamagishi comments that he's just a high schooler; Hirama replies that he's actually a witness. Yamagishi brings up the parasite that had multiple parasites in his body, noting how he must be powerful. He then tells Shinichi not to worry about not being a professional, and asks if Shinichi knows of any type of weapon effective against the creatures. Shinichi thinks for a moment and suggests flamethrowers. Yamagishi begins laughing, then apologizes and says that the idea isn't bad, but using fire in a building would probably cause more problems than they'd solve. He states that even if the dangerous parasite has multiple parasites in his body, he still only has one heart, so the weapons they have should be more than enough.
Back in the present, they start bringing the identified parasite to a different section of the parking lot. Yamagishi says that the creatures can change their heads into blades and have physical abilities far beyond humans, but the most dangerous part is that they can't feel pain. Instead of hitting them in multiple locations of the body, they need to blast away large pieces at a time. They decide to use shotguns, with 16.8mm shots that pierce the body directly and destroy the heart. Yamagishi orders the first shot be taken, blasting the parasite in the chest. Although it initially appears unaffected, it tries to attack the soldier, knocking away his weapon. Hoping they don't shoot it again, Yamagishi watches as the parasite bleeds profusely, writhes in pain and falls to the ground dying. With their first successful kill, they start cleaning up the body and change the next ambush location.
Due to the parasites sensing the female parasite's death, they stop leaving with the group of evacuees, which causes Yamagishi some concern. After receiving a call from Hirama, he is reminded of the possibility of telepathy from Kuramori's report. Hirama decides to send Uragami in. Yamagishi was about to decline, but after hearing about Mayor Hirokawa trying to leave to another area of the building, he accepts the killer's help. As Uragami heads to the building, more evacuees come out in a clump and causing the scanner to be unable to detect them. Uragami tells the soldiers to shoot them. One of the soldiers shoots a human who attempted to run by mistake, causing mass panic. One of the evacuees observing displays fear, and Uragami tells the soldiers to shoot. When they hesitate, the scared evacuee reveals itself as a parasite by attacking the soldier closest to it. Now shooting a self revealed parasite through a window into the City Hall lobby, all the people inside the building begin to panic.
Yamagishi begins firing his gun into the air ordering everyone on the ground, and stating that people who don't comply will be shot on sight. During this time, Yamagishi showed little patience with any citizen who failed to obey a police order. This was seen when a man tried to argue with him but is shot in the chest, killing him. After seeing he was a human, Yamagishi just scoffs and tells the rest of the hostages to follow his orders if they want to go home. Yamagishi calls Uragami over to him and calls him a killer, but thanks him for his help identifying the parasites. Yamagishi then decides to split his squad into two groups, one searches the meeting rooms, while the other sweeps the building. Yamagishi sends Uragami and his handler Katsumata with the meeting room group, stating he's getting the hang of picking out the parasites from the humans. Uragami scoffs at his hypocrisy of calling Uragami a killer.
While searching the building they come across a man who Yamagishi orders to shoot immediately, but his group hesitates. He shoots the man himself, revealing it to be a parasite. When asked how he knew, he says it's because he wasn't in the main lobby. Using these tactics they begin turning the tides in their favor. Though multiple human lives were lost in the process, their main goal was eliminating the parasites.
As Yamagishi's group begins to reconvene with the other squad, they hear excessive firing and he then receives a report that shotguns aren't working on a certain parasite. As Gotou turns the corner he grows two more sets of eyes, prompting Yamagishi's group to fire at him. Gotou quickly darts around the corridor, bouncing from floor to wall to ceiling, dodging bullets, and ripping apart multiple soldiers. He then enters a room to the side, where the squad is surprised to see him partially transformed. He starts jumping around the room again, appearing behind two men and stabbing them in the back through the chest to use them as humans shields. Though one pleads for them not to shoot, Yamagishi says there's no helping them, and to just fire. Analyzing that they're at a huge disadvantage in the corridor, Yamagishi orders his men to retreat to the roof top. To buy time for his men, he shoots two grenades at Gotou. As Gotou walks out of the smoke unhindered, Yamagishi says he's got a bad feeling.
While they are running towards the stairs leading to the roof, Gotou begins killing and picking off the soldiers. After finally reaching the roof, Yamagishi turns around and sees that all his men have been killed. Gotou walks onto the roof noting that his men have been "scattered". Yamagishi asks how he did it. Gotou states that he has armoured his body, but there are gaps, showing him two gun shot wounds. Yamagishi begins shooting at Gotou, who creates a hardened parasite cell shield. Gotou says even though his cells are hardened, it's still dangerous to take bullets head on and so he deflects them instead. Angry at being mocked, Yamagishi shoots another grenade at Gotou, which he easily dodges, clinging to a wall. Gotou says that he's not faster than bullets, but can predict where they'll be fired by reading Yamagishi's movements. Gotou jumps from the wall and rams him in the stomach, then kicks him around multiple times. After getting pummeled by Gotou, Yamagishi asks what he is, and is told "a wild animal".
He then remembers Shinichi saying a flamethrower would be a good choice. Gotou then proceeds to decapitate him and throw his head off the roof, for all to see. His head later becomes the trigger for a nightmare Shinichi has later on.
- The name Yamagishi means "mountain" (山) (yama) and "beach" (岸) (gishi).