Ryo's Appearance in DIGIMON ADVENTURE 02
We see Ryo in a flashback and a dream Ken has about the forgotten time he had spent in the DigiWorld before becoming the Digimon Kaiser. The running time of Adventure 02 in Japan was from April 2000 to March 2001 and WonderSwan's Tag Tamers came out in August 2000. It gave Ken's mysterious past a bit more context.
EPISODE 23 - When a Digivice is Painted by Darkness
air date: September 10, 2000
The episode follows Ken Ichijouji after the death of his partner Wormmon. Ken is in a bad state of mind, not leaving his bed, remembering his late brother Osamu, for whose death Ken keeps blaming himself, and his first contact with the DigiWorld.
Grieving his partner's loss and confused about who he is to himself and to those around him, even his parents, he returns to the DigiWorld, searching for his heart. He comes to the Village of new beginnings, where dead Digimon come back to life in a DigiEgg. He realizes Wormmon could return as well but first he gets some serious scolding from a few baby Digimon for how he treated his Digimon.
Botamon tells him that he cannot change the past and that he has to accept that "the good and the bad are both part of [him]". Ken understands that he will have to accept that he was the Kaiser, that his brother is dead and that he's to blame for Wormmon's death. This is when he remembers how Wormmon had always been with him and has a flashback to a time in the DigiWorld when he's two years younger* (Tag Tamers' age).
This is when we see Ken walking in a desert with Wormmon and Ryo. They get attacked by a Gazimon and Wormmon saves them. Wormmon tells Ken that the Digivice is his not his brother's and that he must not run away from the bad.
Back in the present Ken remembers these words, apologizes to Wormmon and to his late brother for envying him, his crest of Kindness starts to shine and so does the DigiEgg Wormmon is in. The two get reunited in this heart breaking scene.
EPISODE 43 - Attack of the Demon Corps
air date: February 4, 2001
The Digidestined are anxious about their enemy's next move. Ken and Wormmon go to sleep and Ken has another dream/flashback. He remembers the time they defeated Millenniummon in Tag Tamers. It's only him, Wormmon and Ryo in a desert again and Ken looks a bit older than he was in the previous flashback. Millenniummon turns into data, they rejoice but with his remaining strength the Digimon turns his own data into dark spores. Ken pushes Ryo away and the spores hit him in the neck instead. The same thing happens in the end of Tag Tamers, except that they're in the village then and there's also Tai, Agumon and Veemon.
Ken wakes up from this nightmare and asks Wormmon about the injury to his neck. We now know that two years had passed from the end of Tag Tamers:
We know from Tag Tamers that the dark spore that hit Ken was responsible for him turning into Digimon Kaiser. This is what Wormmon says in the end of the game:
In the end of the episode Ken gets kidnapped by Arukenimon and Mummymon and it is revealed that Oikawa needs the power of the dark spore to subdue the children and he scans Ken's neck to get the data and implants it into the children.
The dark spore that hit him when Millenniummon was defeated is guilty of the birth of Digimon Kaiser and also Ken's hazy memory of his first adventures in the DigiWorld. We know from the game that Ken and Ryo knew eachother from the real world and that his first time in the DigiWorld was with Ryo.