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The four WonderSwan games based on the Digimon franchise tell the backstory of Ryo Akiyama. Given how long and fragmented it is, combined with No Export for You, it's useful to have it sitting around in one place. Beware: here be unmarked spoilers for both these games and for the Digimon Adventure and Digimon Adventure 02 anime series.
The events of Digimon Adventure: Anode/Cathode TamerA dying Mugendramon survived long enough to undergo a Jogress evolution with a Chimeramon, giving birth to Millenniumon. With its abnormal strength, it quickly overwhelmed and captured the Chosen Children, and only Agumon escaped carrying Taichi's Digivice. On new years's eve of 1999, he desperately called children around the world for help but only one responded: Ryo Akiyama. Meanwhile, Millenniumon's presence continued to distort the Digital World, and with his power he revived long-deceased enemies of the Chosen to guard his captives. Ryo was drawn into the Digital World by Agumon's call, and before they can do anything the two were confronted by a Kuwagamon or Shellmon, presenting a sharp reminder that the digital world was not a dream. The two won and went to see Gennai, who gave Ryo his task: to recruit a small army of Digimon and lead them to free the Chosen and defeat Millenniumon, as he was the only one who could. Ryo took up Taichi's Digivice; he could not use it to evolve Agumon, but was able to use it to try to purify servants of Millenniumon to join him. Ryo then proceeded to slowly chip away at Millenniumon's follower base, defeating the revived antagonists and freeing all the Chosen but Taichi. He was constantly bombarded by very gruesome threats from each - Etemon expressed a desire to tear Ryo apart, while Vamdemon wanted to decapitate him and present the head to Millenniumon -, a tough order for any eleven-year-old to face. With his forces eroded, the path to Millenniumon's base opened, and Millenniumon himself fell for the first of a ridiculous number of times. Taichi and Agumon reunited, reminding Ryo that despite all they'd been through together, Agumon was not his partner. After being thanked and farewelled by the Chosen and all the Digimon, Ryo returned to his home in the real world.
The events of Digimon Adventure 02: Tag TamersWhile visiting his friend Ken Ichijoujinote , Ryo witnessed the unfolding battle between WarGreymon, MetalGarurumon and Diablomon, watching as the two Jogress-evolved into Omegamon and killed Diablomon. Shortly thereafter, they were visited by a V-monnote , who took them into the Digital World to confront and defeat another Diablomon which was on the loose. Ryo and V-mon killed it easily (without V-mon needing to evolve) at the top of Infinity Mountain, but the Diablomon ruse was a distraction crafted by Millenniumon to draw Ryo back into the Digital World. With one blast, Millenniumon split the entire Digital World in half, destroying the mountain they were on, and separating Ken and Ryo. Shortly thereafter, Ken met his own Digimon partner, Wormmon. Ken and Ryo eventually learned of a "door to Millenniumon's soul", unlockable only by collecting the nine pieces of the Digimental of Desire, and were forced to work separately to gather the pieces, reconnect, and confront Millenniumon together. During their travels, the two were able to converse and exchange items by using Reload Machines scattered across the Digital World. With the full Digimental collected, Ryo and Ken met up and passed through the door to enter Millenniumon's "soul world", and inside confronted the form he took within: Moon=Millenniumon. The two defeated Moon=Millenniumon, destablising and destroying his physical body too. With his last breath, Millenniumon cursed that his power was eternal and unleashed a wave of Dark Seeds meant for Ryo, but Ken took one instead. Ken fell gravely ill, and the two returned home. One year later, Ken returned to the Digital World at the prompting of a mysterious email contact, travelled to the Dark Ocean and bathed his Digivice in its water, transforming it into the black D-3. He then declared himself to be the Digimon Kaiser, the ruler of this world, which is his to do with as he pleases.
The events of Digimon Adventure 02 D-1 TamersShortly afterward, Ryo visited Ken during the illness caused by the Dark Seed, and was prompted by Ken to try a strange online questionnaire on his computer. After completing it, he was drawn back into the Digital World, this time by one of the four Holy Beastsnote . Once there, Ryo was asked to participate in the "D-1 Tournament". He was informed that three of the four Holy Beasts had turned evil, and the sole Holy Beast remaining arranged the D-1 Tournament in response: the winner of the Tournament will partner with the sole Holy Beast to take on the others, either to beat some sense back into them or to defeat them. Gennai gives Ryo a D-Terminal to keep in contact with Ken, and the Tournament begins. Ryo is matched up against other Tamers and Chosen Children also vying for the title (during this, Mimi tries asking Ryo out), and ultimately defeats all of them and wins the tournament. At the end of the Tournament, Ryo was informed that he was the only one not in on the truth of it all: the D-1 Tournament was staged by the four Holy Beasts (none of whom were evil) and all the Chosen as a training program for Ryo, to prepare him to fight an enemy no-one else could ever hope to defeat, a so-called evil god. Ryo was completely shattered by the revelation that he had been used so callously, but goes along with it since he was too shocked to so much as argue back. After completing one last practice round, he was pressured by the Holy Beasts into challenging this enemy: Moon=Millenniumon. Moon=Millenniumon was, surprisingly enough (or unsurprisingly), quite pleased to see Ryo again. Before, during and after, he constantly told Ryo that the two were effectively one and the same, sharing the same power, that they were flipsides of the same coin, light and darkness, and that neither could exist without the other. Moon=Millenniumon then declared that if he was going to die, Ryo would be going down with him. With a massive explosion, Ryo and Moon=Millenniumon are flung through time and space, never to return to the world of Digimon Adventure. Later, Ken laments Ryo's disappearance in a discussion with Gennai, expressing his belief that Ryo is still alive and the two will meet again one day.
The events of Digimon Tamers: Brave TamerRyo was awakened by Monodramon, a small dragon Digimon, in a primitive jungle in the distant past of the Digital World; he found he had amnesia and couldn't remember anything of his past. Monodramon took him to confer with ENIAC, the first electronic computer, the Digital World's first overseer, and the origin of the many different alternate Digital Worlds, able to access them in enclosed timeloops in order to prevent any changes to the timeline. Meanwhile, Millenniumon had since been flung into the future and evolved into its final form, ZeedMillenniumon, and wielding the power to manipulate time and space, was bent on conquering every timeline and dimension. Each of them resisted his power, so he began to assemble an army. He noticed the existence of ENIAC, and so sent a few mooks to test its defenses. In response to the impending attack, ENIAC assigns Monodramon as Ryo's partner to fend of the invaders, but the invader ruse was a distraction: ZeedMilleniumon had instead sent full forces to the Digimon Tamers timeline to capture Culumon, the catalyst of evolution, and Ryo successfully repelled their efforts. Afterward, Takato decided to help him by sending Guilmon and his own red Digivice. As the struggle against ZeedMillenniumon intensified, both Ryo and ZeedMillenniumon gathered ally Digimon from different timelines. Eventually, Ryo finds out the the other tamers that he had fought with in the D-1 Tournament had also been used by the chosen children, while ZeedMillenniumon managed to create artificial humans, the Dark Tamers and their VR partners, who successfully bring down ENIAC. Only then did the "real" first electronic computer appear: the Atanasoff-Berry computer, which had been working as a shadow to ENIAC in order to support him. However, it could only hold off ZeedMillenniumon for so long, and once again, it's up to Ryo to challenge ZeedMilleniumon. In the end, ZeedMillenniumon released Apocalymon and revealed that the reason Ryo always had to borrow Digimon to fight him was because he, Millenniumon was Ryo's true partner all along. However, because Monodramon felt he had also become Ryo's partner, he forced a Jogress evolution between himself and ZeedMillenniumon. The result is a single Digi-Egg, which Ryo returned to the freed ENIAC. As ENIAC returns the gathered Digimon to their home dimensions and partners, he offered Ryo a choice of universes. An exhausted and stressed Ryo chose to travel to a new universe, and although not explicitly shown, he traveled to the Digimon Tamers world, where his Digi-Egg would later hatch into Ryo's "true" partner: the uncontrollable Cyberdramon.
The consequences of Millenniumon releasing Apocalymon, aka "How Millenniumon Fucked Up The Digimon Adventure Universe"You might want to get yourself a drink first, as this could take a while. *deep breath* Milleniumon's release of Apocalymon, the primary antagonist of Digimon Adventure (if one who was never heard of until the end), is obviously what set Apocalymon off at the Adventure universe. His very existence generated chaos upon the universe, from which the four Dark Masters were born. Devimon, Etemon and Vamdemon were also born of Apocalymon's chaos. On the flipside, Milenniumon's actions also caused the solution to the problems he caused - as a result of respectively being caught by Vamdemon and witnessing the Dark Masters' actions in the sky of the real world, Daisuke Motomiya and Iori Hida eventually became Chosen Children. Millenniumon was also responsible for the creation of the Diablomon which came dangerously close to nuking Tokyo, only being stopped by Taichi, Yamato and Omegamon at the last second. This fight was witnessed by people around the world, creating the international chosen children, but three particular observers come to mind: Ryo himself, the aforementioned Ken, and Miyako Inoue who would later become a Chosen herself because of this fight. As shown in his cameo appearance in Digimon Adventure 02, once defeated at the hands of Ken and Ryo in Tag Tamers. Millenniumon launched Dark Seeds out of his body. Ken was infected by one at the time, saving Ryo from being infected; one year later their influence caused him to become the Digimon Kaiser and proceed to abuse the Digital World, devising and creating the Dark Towers which would later play straight into Arachnemon's hands. This culminated in Ken's successful attempts to recreate Chimeramon, intending him to be his perfect partner. Chimeramon, if you'll recall, was one of the base Jogress components of Millenniumon; whether anyone intended this to eventually give rise to another Millenniumon, or even if the unleashing of the spore was something between a Xanatos, and a Batman Gambit, and Ken's creation is the same one that originally jogressed with Mugendramon, is unknown. The fight against Chimeramon resulted in the death of Wormmon, which prompted Ken to undergo a Villainous BSOD and eventually a Heel-Face Turn. In the last few episodes of Digimon Adventure 02, Ken was kidnapped alongside a large group of children by Yukio Oikawa, the host body for Vamdemon, whose spirit had survived his second death at the hands of WarGreymon and MetalGarurumon in Digimon Adventure. Oikawa uses Ken's now-dormant Dark Seed as a basis to copy, and inserts more Dark Seeds into the other children. From this crop of Dark Seeds, Vamdemon regained the power to first kill BlackWarGreymon (just one was enough for that!) and then, with all except Ken's, resurrect himself as BelialVamdemon. Sure, BelialVamdemon didn't get to actually do much, but he could well have had the Chosen not intervened. By the way? Millenniumon started all of this without ENIAC - an omniscient entity - noticing. Damn.