Mitsuo Yamaki is a major secondary character in the Digimon Tamers storyline. He figures within the earlier part of the series as a troubled anti-heroic vigilante who is defeated in his plans by the very forces and tools which he used in his mission to rid the human world of the Digimon, and then becomes a valuable ally of the Tamers in their efforts to save both the Digital World and the Human World from the destruction committed by the D-Reaper.

Role and personalityEdit

While his sinister, black-suited exterior seems to present Mitsuo Yamaki as little more than a basic G-man, those shades hide the mind of an electronics genius. Yamaki was responsible for designing the Hypnos system, which keeps a constant watch over the Earth’s electronic communications network, secretly monitoring and recording information and acts as section chief to the organisation named for his creation. Yamaki apparently also worked on other government projects before. When Hypnos became aware of Digimon existing autonomously within the network, protocols were immediately established to deal with these Wild Ones, as they were code-named, as they began to Bio-Emerge into the Real World. Yamaki, a stiff man of few outward emotions, grew to hate the creatures, viewing them embodiments of chaos and disorder, a danger to humanity, and mere faulty programs. Among Yamaki’s traits is his compulsion to flick open and close the silver Zippo lighter he always carries – it’s generally believed he’s attempting to quit smoking, and flicking the lighter is a possibly-unconscious nervous action caused by wanting a cigarette (due to stress, most of the time). He is involved in a relationship with Hypnos chief systems operator, Riley Ootori.

Later on in the series, he grow to accept there are Digimon that weren't a threat, and is able maintain faith in the Tamers even when things got tough. At nearly all times, Yamaki maintained a very calm persona, rarely displaying any emotion.

Yamaki and the TamersEdit

Dealing with the DigimonEdit

When the Wild One known as Guilmon Bio-Emerged, Yamaki instructed a tracer to be kept on him, to see what he would do. The following day, after Guilmon had battled Renamon in Shinjuku Park, Yamaki headed up the team that examined the battlefield and found a Modify Card lying on the ground.

Yamaki and his Hypnos underlings continued to observe the Bio-Emergence of other Wild Ones, only to have the Tamers and their Digimon defeat them. While on another field mission, Yamaki was called back to headquarters by Tally at the request of his superior, the Chief Cabinet Secretary, to fully explain the nature of Hypnos. The secretary made it clear that Hypnos was to remain a secret from the public, and that all Digimon connection to the Real World was to be eliminated. That night, when Guilmon slipped into a warp between Earth and the Digital World, Hypnos picked up the signal from the tunnel under the park and Yamaki, in direct defiance of orders from above, uploaded his Yuggoth deletion program into the Hypnos system and had Riley target the tunnel with it. The Tamers just managed to rescue Guilmon before the Yuggoth struck.

When a Devidramon Bio-Emerged and flew over the city in clear view of the populace, Yamaki viewed the Digimon’s audacity as a personal insult. There was little action that could be taken, however, but when Growlmon destroyed Devidramon, Yamaki charged Riley with inventing a cover story for the incident.

Hypnos was unable to stop IceDevimon from Bio-Emerging but seemed to have more luck with Musyamon, who was apparently destroyed by Yuggoth. It was not to be, however – Musyamon’s data survived and his voice was heard in Hypnos headquarters, declaring his desire for revenge. He reappeared the following day, and destroyed Yuggoth when it attempted to target him. Yamaki headed for the Shinjuku Guard Rail, the site of Musyamon’s Bio-Emergence, and observed the battle from afar as Takato, Henry and their Digimon defeated Musyamon. Yamaki was subsequently able to identify the two boys by name and tracked them down to deliver a warning – stop playing their dangerous game. Jeri’s arrival distracted the boys and Yamaki departed, only to have Henry pursue him but, unsurprisingly, the boy failed to catch him. That evening, he observed the Tamers’ battle with Harpymon and decided that it was time to end things.

Yamaki confronted Takato face-to-face once more as he and another team of Hypnos operatives captured a DarkLizardmon that Takato and Guilmon had just defeated. During the examination of DarkLizardmon, Yamaki pushed the scientists to increase the intensity of the scan, destroying DarkLizardmon’s form and then instructed them to delete her remaining data. In a subsequent video conference with the government operatives above him, Yamaki persuaded them to allow him to activate his Juggernaut program, which tore open a vortex between the Real World and the Digital World and began pulling Digimon through it, returning them to where they came from. Yamaki and Takato faced off once more but Yamaki simply walked away from the boy... only to be informed that an anomaly had infiltrated the Juggernaut, allowing something to come through from the other side. That “something” was Mihiramon, the tiger Deva, and his appearance on Earth tested Yamaki’s sanity, as he took out his frustrations on Henry, throttling him – only to deem him not worth it. Staggering away, Yamaki returned a short time later, having organised a military strike. Attack choppers opened fire on Mihiramon atop the Hypnos towers, to no effect, but then, Growlmon Matrix Digivolved to WarGrowlmon and destroyed Mihiramon, much to Yamaki’s surprise, forcing him to realize that the kids viewed the situation as more than just a game.

Desperate measuresEdit

Yamaki was, unfortunately, powerless to stop further Devas using the Juggernaut to enter the Real World, and took action by re-uniting the Monster Makers, the original programming team that created Digimon, including Henry’s father, Janyu/Tao, in order to learn more about Digimon and, in doing so, how to destroy them. Prior to this reunion, the horse Deva, Indramon, Bio-Emerged and Yamaki attempted to stop him with an underpowered Juggernaut activation. It failed, but Indramon decided it was time to leave. Subsequently, a strange binary code communication found its way through the Hypnos computers – the Devas communicating with each other – and Yamaki laughed at the simplicity of it. The Monster Makers were then brought together, only to have the meeting coincide with Indramon's reappearance. Yamaki took the chance to explain the mechanics of Digimon Bio-Emergence to Janyu but military strikes on the creature proved fruitless, meaning it was once again WarGrowlmon who stepped up to destroy the Deva.

When the gargantuan pig Deva, Vikaralamon, Bio-Emerged in Shinjuku, Yamaki took it personally – such a massive scale event couldn’t be hidden from the public and it was his responsibility to stop it. The Juggernaut program was once again activated at full power, despite the pleas of Janyu Wong, and Vikaralamon – and the Tamers’ Digimon – were caught in its power. However, the monkey Deva, Makuramon, chose that moment to reveal himself, using the binary code language to communicate directly with Hypnos and warn them that they were getting in his way. He instigated an implosion that crashed the Juggernaut data stream and destroyed a substantial portion of the towers. In a fit of frustration, Yamaki attempted to force some of the technicians to stay but no one in their right mind would listen to him and everyone evacuated. Out of loyalty, and as would later be revealed love, Riley stayed by him but when he realized there was nothing that could be done and silently told her to leave. Yamaki remained in the building as it was consumed by an explosion. Vikarlamon was ultimately destroyed by WarGrowlmon's Atomic Blaster attack with help from Takato lending his strength to him.

Change of heartEdit

Yamaki survived the attack but for him, the outcome was perhaps worse than death - the Digimon had apparently won, and he was fired. Wallowing in self-pity – and cigarettes – Yamaki was snapped out of his funk by Riley, who convinced him that there was still some good he could do. (It was subtly revealed at that point that Yamaki and Riley were living together and carrying on an after-hours relationship.) The Tamers had planned to enter the Digital World to rescue Calumon from Makuramon’s clutches and Yamaki gave them a comm device before they left so they could stay in contact with him. He had changed sides to join the Tamers because he realized they had been able to do what he couldn't and he could do some good by helping them out, he even mentions to Takato how much he envies him.

Unfortunately, the Tamers soon discovered that the device didn’t work in the Digital World – except during a storm created by the hasty re-activation of the unstable Hypnos and Juggernaut system by the Head of National Security. The Chief Cabinet Secretary opposed the move and, when the system began to run out of control, called in Yamaki and Riley to put things to rights. The two of them were the only ones able to pull the system back under control and they were reinstated for it. Subsequently, Yamaki called the Monster Makers and all the kids’ families together to explain the situation to them. During the meeting, he received another message from Takato and Henry.

Not long after, when Janyu took Suzie to play in the park, Yamaki met with him there and they discussed the history of the Digimon project – but were powerless to stop Suzie from being pulled into the Digital World, where she was reunited with her big brother and the other Tamers.


Things took a turn for the worse when the Chief Cabinet Secretary restarted the Hypnos program again with untrained replacement personnel. Hoping to use the Juggernaut system to finish the job that Yamaki had started, they reactivated the digital weapon and it began to wreak havoc once again on both the Real and Digital Worlds. With the Juggernaut Core still unstable after the attack on Hypnos HQ by Makuramon the system ran out of control and put the Tamers in the Digital World in grave danger. Yamaki and Riley quickly showed up to stop the system before any serious damage could be done. Yamaki - and by proxy, his team - were fully reinstated after saving everyone and shutting down the Juggernaut again. With his government privileges restored he and the Hypnos were free to use all their resources to help keep track of and aid the Tamers in the Digital World.

Events in the Digital World proceeded apace, as the Tamers battled Beelzemon and Zhuqiaomon, lost Leomon and discovered Biomerging. Meanwhile, on Earth, Yamaki made all of Hypnos’s facilities available to the Monster Makers to construct an "ark" that would bring the Tamers home. Guided by Yamaki via the comm device, Takato led the Tamers back to the surface layer of the Digital World, and onto the ark. An attack by the deadly D-Reaper on the Hypnos central processor almost derailed the rescue attempt but the Monster Makers were able to harness the processing power of computers all over the planet to make it work. Yamaki was so impressed by their sheer skill and ability that he even became calm enough to set down his lighter. The ark materialised in Shinjuku Park and the Tamers with it.

Facing the D-ReaperEdit

The fight was not over. The D-Reaper had pursued the Tamers back to Earth via Hypnos and engulfed a large portion of Shinjuku in its mass. Yamaki, Riley, Tally and the Monster Makers were forced out of the Hypnos towers and had to operate out of one of the organisation’s vans. Later, they established a temporary base in the Tokyo Opera Tower and Yamaki, Riley and Tally went to the Matsuki bakery to meet with the Tamers and then watch as they battled one of the D-Reaper’s Pendulum Feet Agents.

As everyone made their own plans for dealing with the situation, Yamaki was offered a moment to reflect on his actions. Riley realised that he was blaming himself for everything that had happened, and tried to console him, telling him that he was just the first man who had tried to control the network – the uncontrollable. Yamaki laughed, telling her he had no intention of being the first in a line of fools. During a subsequent battle between the Tamers and more of the D-Reaper’s Agents, Yamaki was shocked to receive a message on his comm device from the ark, asking for instructions – the Digimon core code used to create it had made it self aware, and it now resided in the Digital World, low on power. Yamaki turned to Daisy for her help in establishing a connection with the ark, and she succeeded, redesigning it into Grani. Later, when another battle broke out, Riley was able to hack into the D-Reaper’s signal and utilize its ability to view through layers of data, revealing to everyone that the Tamers had actually merged with their Digimon. Yamaki was overcome with shock at the revelation and blamed himself for allowing the conflict to reach this point. He quickly regained his composure, however, and hit upon the idea of using a satellite to directly communicate with Takato and direct him to use Grani’s Yuggoth Blaster on the D-Reaper.

The end result of the battle was that the D-Reaper evolved into the Mother D-Reaper, forcing everyone to retreat to the Tsukuba Communications Research Centre. There, Yamaki co-ordinated Operation: Doodlebug, the final plan to defeat the D-Reaper. When the Tamers returned to Shinjuku to face off one last time with the D-Reaper, Yamaki, Janyu and Dolphin directed Henry to activate the Juggernaut within the D-Reaper’s portal between the Real World and the Digital World and to reverse the flow of the vortex, pulling in the D-Reaper and reverting it to it’s original harmless form – but Yamaki was forced to conceal the truth, that the Juggernaut would also return the Tamers’ Digimon to the Digital World. In a final voiceover he commented that perhaps humans and the D-Reaper were not as different as we liked to believe, viewing the D-Reaper as "misguided program that thought it was doing the right thing", but since the D-Reaper was defeated by humans and Digimon coming together perhaps there was hope for humanity.

Some time later, as Takato reflected over the events that followed the D-Reaper’s defeat, Yamaki and Riley were shown being served in the Matsuki bakery.


After the tamers return to the digital world in search of their partners, he and the rest of Hypnos and the Monster Makers prepare another Ark to retrieve them. They also create a machine that separated Leomon's data from Impmon, thus returning Jeri's partner.

As more tamers appear and events continue to develop, Hypnos, and Yamaki specifically, take drastic steps to protect and assist the tamers so that they will continue to handle Wild Ones and other problems. He understands now that these children and digimon are the most effective force to handle digital threats. He finds housing for Ryder and Mari (and Chiyo before her betrayal). He even alters damage reports and take care of those situations personally so that the government will have less reasons to frown upon the tamers.

When Rayleigh Thompson was assigned to Hypnos, Yamaki treated him like an assistant. He did not like the man and made no attempts to disguise that. When Rayleigh was promoted to a position above Yamaki, he went even further to insult and humiliate the blond-haired man. The relationship between these two continued to deteriorate swiftly until Yamaki actually punched Rayleigh. Yamaki regained his former position at Hypnos when Rayleigh was promoted further up, just below the Prime Minister in power. After the Prime Minister's assassination, Rayleigh became the acting PM and invaded Hypnos with a small force of police in order to arrest Yamaki. He escaped, however, and made his way to the park with Riley, Tally, and Eiichi to meet with the other tamers. The group was ambushed by Rayleigh's forces, but managed to make it to the Digital World before anyone was killed (except Ryder again) or captured.