Kosuke Kisakata

Kosuke Kisakata is the leader of the Sons of Chaos and is a legendary hacker. He is known by the name Tartarus (タルタロス). Very few people know his actual name and he usually uses fake accounts to hide his identity. His partner is, though when he was a teenager he was partnered to a.

Etymologies
Name used in Digimon Seekers.
 * Kosuke Kisakata

Digimon Seekers
During his time at University, Kosuke studied and worked under Tomonori Ryusenji. During that time, he dated Tomonori's daughter — Saya Ryusenji. Ryusenji Laboratories was able to vault to the forefront of Digital World research shortly after its discovery due to the hard work of Tomonori's students: Kosuke, Saya, and Shuu Yulin.

One day, when Yulin arrived at Ryusenji Laboratories and delivered the mail to the professor, who was busy analyzing some code. He complains about the code itself being awful, expressing shock that it was probably written by Ryusenji himself. As they discuss Yulin's request for a proper server room, the campus being transferred to Denrin, and the new investments in their field around the world, Ryusenji lets her know that since the campus was going to be moved, all requests such as the server room were rejected since it'd be seen as a waste of money. Yulin questions if they will have to go overseas, prompting an annoyed response by Kosuke, who once again had slept in the office. As Yulin calls him out for his habits, Saya arrives with food for everyone and a change of clothes for Kosuke.

As the group eat the food Saya had brought them, Yulin scolds Saya for coddling Kosuke too much though she fires back about how she is his wife and not his mother (though they were still dating, not married ). Tomonori laughs at the group's interactions, with the group moving on to talk about their research. Since Digimon didn't live very long, research wasn't going well and they figured they needed to find a way to breed and raise their own Digimon to make studying them easier. Saya mentions that if they really are living creatures they could raise them like pets, though Yulin points out she didn't feel that way herself. As they muse that extra-terrestrial life was actually on the Internet the whole time Tomonori laments that they will need money to be able to continue their research — something everyone is able to agree to. Fully devoted to this cause, Tomonori announces that he is going to start a company and as they doubt his ability to Kosuke brings up that the University had money set aside for start up companies which they should be able to use for this end if they were able to convince them to back the project. They decide Yulin should make the pitch since she was trilingual and good at presentations but Yulin herself was worried someone with Chinese ancestry doing so would ruin the entire plan. She winds up agreeing to do so as the others let her know that it didn't matter and she was the only one who could do it.

Some time later, having found a new office, Ryusenji Laboratories prepared to start their move. Kosuke tells the others the they wont see much of him as he would be "fighting on the front lines", with Saya disappointed with the fact he didn't have enough credits to graduate yet. Kosuke muses that it doesn't really matter as he doesn't mind staying at the University for a few more years with Tomonori saying he will hire him regardless of if he graduates. Saya protests, saying that's not something a professor should say — but he simply laughs saying maybe she's right.

Tomonori Ryusenji and his students were able to get permission to use a warehouse within the Denrin District as a temporary base for their work despite the fact it was going to be destroyed for redevelopment soon. The group redecorate and replace the name plates with it acting as the headquarters for not just the University but Ryusenji Electrics as well. Tomonori was very happy, since he was now running his own business — even with the massive debts he had incurred setting it up. They had managed to set it up with a mix of University funds, money from the government, and Tomonori's own money — the biggest amount coming from Tomonori himself. Now they had managed to set things up, the group were ready to use Digimon to unlock something significant for humanity.

Some time later, Ryusenji shows off the fruits of his labor — the Digimon Dock. He intended to use it to record the actions of the Digimon, though none of his students were impressed with it. Since the Digimon die fast on their computers, he wondered if he could recreate their habitat as closely as possible within something else — hoping the Dock would be able to help with that. He decided to make it a toy, as he wanted children all over the World to be able to become friends with Digimon. When he refers to the Digimon as humanity’s irreplaceable companions, Yulin becomes enamoured with the idea and when Saya notices this she gives her a Digimon Dock. Yulin is surprised to see a Digitama on it, with Saya and Tomonori explaining they had found it on the Internet and that the Dock was able to display the data itself instead of as a series of numbers like on their computers. The group are amazed at the sight, and become even more excited when the Digitama begins to move. Thinking it was going to hatch, Tomonori snatches it out of their hands as he wanted to see.

Tomonori was promoted to being a full time professor at the University. In addition, a big tech company invested so much money in his work it allowed Ryusenji Electronics to become a proper business. Tomonori was able to hire more staff and as the company got bigger they realized they needed to move on from the rented warehouse.

At some point, Kosuke proposed to Saya and she accepted.

Saya and Yulin discuss and celebrate the fact that Kosuke had proposed to her. They then discuss the new employees at Ryusenji Electronics and the fact that the company would need to go public to pay back their investors since thus far they hadn't actually made any money. Kosuke joins the conversation, joking that Yulin was trying to steal Sara from him, and shows off a VR headset. He puts it onto Saya's head and tells her that she is currently seeing through a Digimon's eyes and can see what it can see — though it was a recording rather than live.

Wanting to one day explore the Digital World, Tomonori continues to work even harder. Wanting to experience all five senses whilst there, he began researching DigiCores in the hopes he could transfer human consciousness into it and allow his goal to come true. He found it was possible too, as other scientists were researching ways to convert the human consciousness and memories into data. There were even machines that would allow blind people to access sight, or deaf people to access hearing though the government had made it so that they couldn't move past research for around 10-20 years. Tomonori figured however, that since the Digital World was a different World, those rules wouldn't apply which lead him to focus his time on this endeavour and would focus on it even if it took him the rest of his life. To make sure no one would find out and stop him, the company researched it in secret.

Some time later, Saya would become the first person to do a Mindlink — linking her mind with a Digimon — though the effects on her body were bad with her vomiting and defecating herself in front of everyone. As she did more and more Mindlinks her body got used to it and the side effects would be less severe. Upon coming too, she looks into her Digimon Dock and praises her for its hard work. Yulin and Kosuke had also been given their own Digimon partners, with Kosuke's turning his life around since he had to make sure he was able to properly look after.

All three of them would go through many Mindlink tests though thus far had only explored their Digimon Docks. To be able to explore further than that, and reach the Digital World, they would need better computers. They would also need to learn how to control their partners, since if they lost control they would go passed the Mindlink limit and be unable to return to their bodies. As they discuss these facts, they also theorize what the Digital World would actually be like.

Kosuke was in charge of the development of the Digimon tools and would send Digimon out to the Digital World to explore. Around 80% of the Digimon would return alive, giving them lots of useful information. Tomonori muses that it would be impossible for such tests to be 100% successful, comparing it to space mission in the past — this making the students uncomfortable since he brought up instances of astronauts failing to come back especially since they would soon be joining the Digimon on their travels via Mindlinking. They would refer to said upcoming Mindlinking project as the Tartarus program.

The night before beginning the Tartarus program Yulin, Saya, and Kosuke visit the plot of land on which Tokyo University of Electrical and Computer Engineering was to be built in the Denrin District and look on in amazement that such University was going to be built there. As they take in view of its current appearance, before the building would be made, Saya makes a wish that everyone would still be together in ten years — including their Digimon partners. She shows off the area to her BlackAgumon, by pointing her Digimon Dock at it, and makes another wish that the Digimon will be able to live alongside the humans. Yulin is confused at the fact Saya was talking to her BlackAgumon, since they can't talk back, but she tells them that BlackAgumon had actually been talking to her during their Mindlinks — a fact that shocked Yulin. Realizing what she had said, she tells Yulin not to tell anyone as she was scared she would be seen as crazy and be removed from the project. She tells her that BlackAgumon would call out her name and tell her she's glad she came to play and this leads Yulin to think Digimon might actually be dangerous and that they were actually the ones discovering humans, not the other way round. Saya points out it was too late now though, and that their Worlds were now destined to collide. She also tells Yulin she is considered the most important member of the team as Saya was Tomonori's daughter and Kosuke was just a younger version of him. In addition, she was considered to be the only one to be to stop them should they mess up. Kosuke joins them, giving them warm drinks, and though he tries to get them to leave due to the weather Saya insists they stay longer. Yulin leaves, returning to the car to warm up, and with the couple now on their own, they act like love birds which irritates Yulin and causes her to take a picture of them.

The next day, Yulin, Kosuke, and Saya Mindlink to their partners. They have their partners board the Tartarus probe, a digital observation vessel so that they could begin the Tartarus program. The Digimon search the Digital World on their own virtual vessels with their partners controlling the vessels using Kosuke Kisakata tools. Due to the weak technology being used, the groups visibility was low but despite this they followed the path set for them and continued on their journey. As they explored, they were attacked with Kosuke and Yulin having no clue what had attacked them. Yulin and Kosuke figure BlackAgumon had been the attack target, and were unable to do anything as Saya's unit vanished without a trace. Kosuke blamed himself, as he had been the one to push for her to be a one woman rescue team, and regretted it wishing he had been doing it instead so that she would've returned safe in place of him. Tomonori forces Kosuke and Yulin to retreat as he considered it far too dangerous to have them try and rescue his daughter. The Tartarus probe was damaged and and Tentomon were injured and as such he had them withdraw as the chance of all three of them falling into DMIA was too high. When they logged out, Kosuke cried as he held his unresponsive fiancée. Her consciousness had fused with BlackAgumon's DigiCore, and BlackAgumon's whereabouts was unknown. Now in a coma, her body was sent to the local hospital with no hopes of her ever waking up.

Over the following years, Tomonori made sure to keep Saya's body in a stable condition. He sent her to a hospital in the United States that specialized in coma patients then continued the Tartarus program and Mindlink tests in secret — though without the help of Yulin and Kosuke as he didn't want to put them in any more danger. Despite losing Saya, most aware of what happened considered the project a success as they had managed to capture footage of the Digital World and their programs allowing control of their Digimon worked as intended. Yulin and Kosuke didn't feel this way though, and considered it a failure. Kosuke cut contact with Yulin, and dropped out of University — though still kept in contact with Tomonori.

Many years later, and the Digimon Dock was a success. Many people had befriended and raised Digimon and and it did indeed allow them to live longer. More and more sponsors poured money into Tomonori's research and with more money, Tomonori was able to increase his influence within the University. The extra money also allowed the University itself continued to grow.

Many years after that, some time before leading the group known as the Sons of Chaos, Kosuke lead a group hackers — with those hackers later forming the core of the SoC. Kosuke and his Hacker friends, Mindlinked to their partner Digimon, found themselves in the Gateway within the Wall Slums, looking to hack into an impenetrable firewall. It was going well but took a turn for the worst when a the system administrator discovered them which lead to the admin sending a group of gatekeeper Digimon to attack them. and Marvin Jackson defeat most of them but they are quickly replaced with more. As they had already been Mindlinking for a while, Marvin tells Kosuke he wasn't going to be able to hold off the horde for much longer. All of the hackers, sans Kosuke, had their Digimon at the Ultimate level, but even that wouldn't be enough to defeat thousands of Digimon. Kosuke declares it was time for Phase 3, and had Dorumon digivolve to the Ultimate level. DoruGreymon charges up its Metal Meteor attack, which has Marvin realize that Kosuke wasn't playing around. He has the other hackers scatter with Megadramon and the other partner Digimon firing attacks at the gatekeeper Digimon in the air. Kosuke begins a hack, which causes all the gatekeeper Digimon to be sucked under the Metal Meteor. They try to resist but to no avail, with it landing on them and killing most of them. Marvin and the others take out the survivors but when their hacking reaches 30%, it stops. Kosuke asks what happened, with DoruGreymon telling his partner that they could only get to 30% with one Prototype Digimon. Not just that, to be able to successfully hack into the system they would need three Prototype Digimon at the Ultimate level with the three needing to consist of Vaccine, Data, and Virus Digimon. Kosuke thinks and figures out that with DoruGreymon a Data Attribute and Shuu Yulin's a Vaccine they would need one more Digimon — a Virus. The gateway had been there even before the Wall Slums themselves, which is why it required such Digimon. Beyond the gateway was the source code, and Kosuke wanted it so that he could change the World. As they muse over Kosuke's goals, one of the shows up and — knowing full well they had no chance against a member of the Royal Knights — tells everyone to flee.

He eventually went on to create the hacker group known as the Sons of Chaos.

Though Kosuke sets up the SoC's missions, he rarely gets involved himself.

Obsessed with finding BlackAgumon, he puts up a mission with a 1 million DC reward for anyone that can get him one.

When Eiji applies to join the the SoC, Tartarus does an interview with him (Eiji unaware of this fact, as Tartarus was using a generic account). Tartarus tells him that he can only join if he is a good code cracker, to which Eiji asks if he is whether that would allow him to meet Tartarus. Tartarus finds this curious, though Eiji insists that since Tartarus is so well known anyone would want to meet them. Tartarus tells him that if he is really good, one of the higher ups would eventually contact him and allow him to meet Tartarus but that very few members had actually managed to meet him. Tartarus sends Eiji an exam, and tells him to complete it. Eiji finds that the mission is named "A Wolf at the Back Gate", which he finds odd since he has, though doesn't mention that fact. Tartarus tells him that nine out of ten people who do the exam lose their Digimon, and they would not insure his Digimon for him or pay compensation should he lose his Digimon. Eiji wasn't deterred though and proceeds to Mindlink.

Tartarus waits for Eiji on top of a building whilst Mindlinked to, who hides under a cloak to hide its identity. Eiji, Mindlinked within Loogamon, arrives two minutes late which causes Tartarus to dock him two points on the mission. Eiji suddenly appears next to Loogamon, which surprises even Loogamon. Eiji explains he had remembered he was told he could access commands on virtual monitors by thinking of doing it and having seen a button saying hololize simply thought of pressing it which allowed him to hololize next to Loogamon. Tartarus was also surprised and tells Eiji that his mission is to create a map of Ninth Avenue for them, and that he just has to walk around the area with the provided map tool as it would auto fill the information for him. Loogamon laughs, and informs Eiji that the mission was perfect as it had something it needed to do in Ninth Avenue anyway. Tartarus wishes Eiji luck on his mission, hoping he would be able to pass and become a member of the group.

As they watch Eiji and Loogamon leave, Dorumon tells Tartarus it was surprised Eiji was also Mindlinking to which Tartarus expresses surprise at not just that but the fact that Eiji had already shown up with his limits unlocked. It wonders if Loogamon had noticed they were also using Mindlink then asks why Tartarus had given the Demon Wolf of the Castle of Nine Wolves such an easy mission. The SoC member realizes Dorumon was right and contacts the DigiPolice, telling there where Eiji was.

When Eiji and Loogamon arrive at the Castle of Nine Wolves they are intercepted by the DigiPolice who ask to talk to him on GriMM. Turning down the request would be seen as going against the DigiPolice but Eiji had no choice in the matter as his means of communication during the mission was in the hands of Tartarus who proceeds to turn down the request.

Dorumon and Tartarus, still Mindlinked, watch in awe as Eiji and defeat the DigiPolice. After the two of them leave, Dorumon chastises Tartarus for telling the DigiPolice about Eiji, though he just muses that he was right about how powerful Loogamon was. He notices that Eiji had completed the mission having sent him the map data, and decides to accept him as an official member of the Sons of Chaos in the hopes that Eiji will be able to lead Loogamon into become an even more powerful Digimon. The wind then blows off Dorumon's cloak.

The members of Operation Infinity meet up in Marvin's SoC GrimM room, with there being eight users there total. The monitor displays the mission, which is to hack X Nation's server. Hackers from X Nation had performed lots of cyberattacks and had even recently stolen billions worth of cryptocurrency. Since the SoC use cryptocurrency they couldn't turn a blind eye to them doing such a thing, especially if it caused a major shift in the World's financial powers. The SoC plan to destabilise X Nation's leader as well as get revenge on them, as they had done crimes using a Machinedramon and framed the SoC. Tartarus arrives, introduces himself as the leader of the SoC to the new members, and then tells them to render their server useless and capture their Machinedramon.

The SoC's plan to get revenge on X Nation for framing them for crimes they didn't commit by hacking into their servers and stealing their Machinedramon ends in failure after Leon Alexander and defeat them, and also kill the Machinedramon. Shortly after the failed mission Dorumon and the Hololized Tartarus, who was using his interviewer account, visit Marvin Jackson is in his room in the Sons of Chaos GrimM channel. Dorumon asks how was, with Marvin stating that it was okay but would be out of action for a while due to the injuries it had sustained at the hands of Kazuchimon. He tells Dorumon that Eiji, the hacker that Tartarus had personally scouted, had saved him and wonders if he should retire having lost that badly. Social media had become ablaze with rumours about how Judge had single handedly defeated the SoC, with the SoC themselves having leaked this information. Dorumon and Tartarus tell Marvin they would not let Judge get away with it and would get revenge against him as he poses a threat against not just the SoC but the entire Crack Team. Marvin asks if the duo are going to go ahead with their plan for an all out war, with them confirming that even with the loss of Machinedramon the final piece of the puzzle had still fallen into place. They discuss Eiji and Loogamon, saying that they had only seem him digivolve to Champion and as such didn't know whether they could trust them to stick with them till the bitter end without seeing their full power. He asks Marvin if he is still dedicated seeing Tartarus's plan to the end, to which Marvin confirms he is.

Tartarus sends Eiji a message, praising him for his work during the attack on X Nation's server with Tartarus stating he wants to hire Eiji again in the future. He asks Eiji to get revenge against Judge for interrupting their mission since failing to retaliate would lower the image of the SoC. He allows Eiji to determine Leon's punishment, with Eiji stating he would only do so under those conditions. He comes to the agreement that if he could get Leon to agree to never interfere with the SoC again, they'd let him get away with just an apology. He otherwise, would have to take Leon out. Eiji is unable to come to an agreement with him, leading to a battle in which Leon overuses his Mindlink causing him to become DMIA and fall into a coma. Presumably unaware that of the  had shown up in the middle of their battle and Leon had sacrificed himself to save Eiji from it Tartarus sends Eiji a message congratulating him on taking out Leon, telling him he was the hero of the Sons of Chaos.