Akiyama is the only human character to span two continuities. He is partnered with Cyberdramon. He originates from a series of games for the Wonderswan game system, appeared in a few cameos, then was retired in Digimon Tamers.
He made his first appearance in Tamers when Kenta, Kazu and Rika were cut off from the others while in the Digital World. Later, he would help the others escape to their world, and follow them as well. While in the Real World, Cyberdramon regressed into Monodramon, and apparently was freed from the violent virus he suffered from. He later bio-merged with Cyberdramon to become Justimon, and, as Justimon, he helped out in the battle against the D-Reaper.
Ryo is one of the two human characters to appear in both the Adventure series and in Digimon Tamers (the other is Tai, but it was more of a cameo). It was decided that Ryo was to be included into Digimon Tamers because of his immense popularity at the time as the main character of many Digimon video games. The final Wonderswan game to feature Ryo, Brave Tamer, serves as link in the chain that connects him to both the Adventure Series and the Tamers Series.
Ryo showed up as Justimon during the Parasimon fight, after Rika, Henry, and Takato Biomerged with their Digimon. However, there are too many Parasimon to fight off and they were losing until Gallantmon Crimson Mode destroyed the hole the Parasimon were coming out of. Afterwards, Ryo and Cyberdramon attended Rika's birthday party.
In the Digimon Adventure V-Tamer 01 manga, Ryo and Monodramon briefly team-ups with Taichi, Zero and Gabo, when Zero creates an interdimensional hole with one of their sneezes. A Demon's assassin, Vikemon, appears to challenge Taichi, and Ryo wants to fight with him, but Taichi stop him, and both boys starts to fight. Vikemon, whose weak point was passionate people, is defeated and Ryo and Monodramon return to its own world.
Ryo suddenly finds himself in the middle of a forest and confronted by Agumon, Tai's Digimon partner. After Agumon explains that Ryo was summoned because a powerful evil Digimon called Millenniummon captured the DigiDestined, then warped the very essence of time and resurrected previously defeated evil Digimon such as Devimon and Myotismon, Ryo quite understandably finds it all hard to believe and thinks he's having a nightmare from too many video games.
He quickly realises he has no choice but to help, however, and thus begins his quest to rescue the DigiDestined and defeat Millenniumon. Along the way, he befriends a number of Digimon allied to the DigiDestined, including Leomon, Andromon, Piximon and many others. He also encounters villains previously defeated by the Chosen Children beginning with Devimon, then Etemon, Myotismon and the Dark Master Piedmon before confronting Milleniumon. (Depending on which game, he takes on the form of one of his two components, either Machinedramon or Kimeramon). After a fierce battle, Ryo and his Digimon comrades are able to vanquish the villain and rescue Tai, the last of the DigiDestined being held prisoner. His task done, Ryo bids his new friends a tearful farewell and returns home at the same time he left, where his parents note he seems to have grown up a bit.
Some events that take place in this game contribute to Ryo's apparent mental breakdown in a later game. For example, each of the evil Digimon are very clear as to what they want to do (for example, Etemon threatens to rip Ryo to shreds while Vamdemon intends to cut off his head and offer it to Millenniummon). Also, unlike the Chosen Children who were able to draw strength from each other, Ryo was alone for the most part (not including the Digimon he recruits). When he realizes during Digimon: D-1 Tamers that the Chosen Children and Sovereign/Holy Beasts were using him as a tool, lying to him in order to train him to defeat Moon-Millenniumon once and for all, he proceeds to emotionally shut himself out and refuses to speak to anyone after that. This is observed by Tai immediately after the betrayal is exposed, and later by Monodramon, who observes how cold Ryo is towards him.
Ken woke up in a forest clearing to meet his digimon partner, Wormmon. Ken searched for Ryo at first, but Wormmon took him to Gennai, who told him that Ryo was on the other half of the world. During this, Gennai said that they must set up a mirror site, and that Ken is a Chosen Child. But he doesn't want to be a Chosen Child, Ken says. Only after Wormmon guilt-tripped him did Ken accept. Ken was then told to clear out the beach side of the continent, where he picked up the pieces of Digi-Eggs, cleared several dungeons taking the form of a ziggurat, an Egyptian pyramid, a Mayan pyramid, and a Black Ship. Once he returned from his mission, Gennai and Piximon set up a Reload Machine that would allow Ryo and Ken to contact each other and share items. The scene switched to Ryo, who must follow on Ken's path and jump ahead to another dungeon to clear. Thus the name, Tag Tamers.
After many battles, DNA Digivolutions, and archaeological digs for more Digi-Egg pieces, Ryo eventually arrived at Millenniumon's floating continent. Millenniumon, speaking in archaic Japanese, chuckled and said that he could not be defeated by just one power. The key to his power cannot be unraveled, and the door will never be opened. He then disappeared, and a single locked door turned up outside of Primary Village. Back at base, Piximon said that Millenniumon still had several traps on Ken's side of the world, so the scene switched to Ken.
Along the route to Millenniumon's dungeon, an Elecmon told Ken that the locked door led to Millenniumon's heart, then wonders if Millenniumon has a heart at all. In a sharp contrast to his more polite interaction with Ryo, Millenniumon was outright rude to Ken and insulted him before disappearing.
The key to the locked door was the completed Digi-Egg of Desire, which only reacted to Ryo. Once it opened, Ryo, Ken, and their digimon were absorbed in darkness and taken to Millenniumon's world: a barren wasteland with a gigantic metallic sphere hovering over it. The sphere was Millenniumon's castle. Millenniumon welcomed Ryo warmly to the world created from his mind. To Ken, his voice dropped coldly as he said that everything that happened there happened also within him. Ken's power is limited there. Then he turned again to Ryo, chuckled, and invited them to challenge him, even though they trembled in despair.
The battle that ensued destroyed Millenniumon's body, even as he laughed in his defeat. With Millenniumon's defeat, the Digital World, Taichi and Agumon returned to their normal states. Then Millenniumon returned only to disperse his Dark Spores. Ken shoved Ryo out of the way of one, only to get it in Ryo's stead. Millenniumon's spirit chuckled. He can never die, he said, and as Ryo's power is eternal, so is his. He then disappeared.
In the scene afterwards, Wormmon said that Ken became sick and increasingly cold to him. The epilogue was meant to show Ken's eventual fall into the Kaiser persona a year later, and in it Ken said that he was different from Millenniumon. He was the Digimon Emperor.
Ryo was then dropped off in the Digital World and told by Piximon that they need Ryo's power again. Understandably skeptical, as he was only called to the Digital World when Millenniumon was on the rampage, he asked why. The Harmonious Ones that ruled over the Digital World called a tournament, Piximon replied. Gennai added that other children were called in to participate and urged Ryo to go to the arena.
Once there, a group of tamers (none of whom have never appeared in any of the televised shows), gathered around to bully Ryo. One said that he expected better of someone who defeated Millenniumon. Another asked whether Ryo can remain undefeated. Unwilling to stand up for himself, Ryo said nothing as the other tamers closed in. Before a brawl could commence, Piximon appeared to break it up, said that the tournament was to find the greatest tamer and that they should all get along better than this, and sent Ryo back to Gennai. At Gennai's house, Ken greeted him from Gennai's monitor. Gennai set it up so that Ken could still communicate with Ryo through the computer and D-Terminal, since Ken was still sick and thus stuck in the real world. ToyAgumon turned up just then to announce that the Harmonious Ones would address them all.
The Holy Beast introduced himself (as the one the player chose during the quiz) and explained that the other three Harmonious Ones became evil and were preparing to defeat the good one, before going on to their individual plots to eliminate each other so that only one could rule the Digital World. The child that would win the tournament would be declared the strongest and chosen to partner up with that Holy Beast to bring his brethren to their senses. Piccolomon then explained the tournament and that it was in five stages. The competitors were to train before tackling each stage. After Wormmon, Agumon, and Veemon offered their aid, Ryo trained and went on through each stage and dungeon that opened afterwards. Grade 5 was headed by Izzy Izumi, Grade 4 by Willis, Grade 3 by Mimi Tachikawa (who asked Ryo to go out with her on a date after the tournament was over), Grade 2 by Matt Ishida, and Grade 1 by Tai Kamiya. Ken offered advice and suggestions through the D-Terminal.
For the first three stages, nothing really eventful happened. It's afterwards that things went along a downward spiral: the Digimon Graveyard dungeon required Ryo to sacrifice a digimon so he could escape. After Ryo won the final grade, Taichi revealed that although neither of them knew it originally, the D-1 Tournament was a hoax. Ryo had already been selected to win anyway, it was all training so that Ryo would be strong enough to take on Moon-Millenniumon, the bodiless spirit form of Millenniumon, and no one else could defeat the evil god. Even as the other Chosen Children apologized for deceiving him, Ryo remained silent and closed off against them. He would remain this way for the rest of D-1 Tamers and into Brave Tamer. Nonetheless, Ryo marches on through the final practice dungeon before being sent to Moon-Millenniumon's floating island.
This time, Millenniumon's lair is in a crystal tower on the floating island. Moon-Millenniumon said that he had been waiting for Ryo, he had revived to challenge Ryo again, and no Digimon is a threat to him anymore. Not even the Holy Beasts can defeat him. But Ryo is different, he said, for Ryo is the only random element that makes the impossible possible. After his defeat, Moon-Millenniumon reveals that he and Ryo are Yin and Yang, that one cannot die without the other. If he was defeated, so was Ryo. He invited Ryo to fly with him through the worlds of time, and the floating island exploded.
From that point on, Ryo was considered dead in the Digimon Adventure universe. Despite constant searches, he was never found and everyone but Ken gave up hope.