Oikawa is considered by many to be the most tragic character in the series. He is one of the few human adults to have a knowledge of the Digital World.
Pre-Digimon and between Digimon Adventure & 02
Oikawa was born around 1973. As a child, he was an extreme loner; mocked and shunned by classmates. A boy around the same age, Hiroki Hida, became his only friend.
Hiroki was obsessed with the idea that he could communicate with the Digital World through old video games and he shared the same obsession with Yukio. They both ended up communicating with the Digital World, spending hours and hours on the video games talking with the Digital World.
Hiroki's father, Chikara, became very worried about the obsession his son had. He forbade his son from playing video games and tried to separate Hiroki from Oikawa, fearing that they were spending too much time together. The two were heartbroken but remained close friends; they made the promise to each other that, one day, they would go to the Digital World and they started sharing their dreams and never told anyone else about it.
In adulthood, Oikawa worked with Ken Ichijouji's father as a computer programmer, while Hiroki became a police officer and started a family. Evidently, the two continued to study the Digital World; in 1995, when the Highton View Terrace (Hikarigaoka in the original) incident occurred, Oikawa went to investigate and worked alongside Nancy Takaishi (Natsuko), T.K. Takaishi's mother. There, Oikawa learned that Highton View Terrace served as a gateway to the Digital World. In 1999, Hiroki took a bullet for an important political figure and died from his wounds. Oikawa was utterly devastated by the death of Hiroki, though he still continued his investigation of the Digital World in being of being emotionally distraught that Hiroki had broken his promise.
On August 3, 1999, Oikawa was present in Tokyo Harbor when the DigiDestined defeated Myotismon and the Digital World appeared on the sky, raising a framed picture of Hiroki to the sky, crying that he wanted Hiroki to see their dream realized. Seeing the DigiDestined returning to the Digital World, his travel to the other world attracted the disembodied spirit of Myotismon who offer to grant his wish in order for letting him possess him. Though Oikawa had no recollection of his meeting with the spirit, he benfited from learning the means to create two Digimon to serve: Arukenimon and Mummymon, created by combining data with his own DNA. While they could travel to the Digital World, he still could not and finds a loophole by weaken the boundaries between the worlds to accomplish this. After seeing Ken Ichijouji in the funeral of the boy's older brother and sensing the Dark Spore in the boy, he manipulated Ken into becoming the Digimon Emperor to do his dirty work.
Digimon Adventure 02
As the Digimon Kaiser, Ken constructed Control Spires which began to weaken the boundaries and upset the balance of the many worlds. But when the DigiDestined defeated the Digimon Emperor and began demolishing the towers, Oikawa sent Arukenimon and Mummymon to battle against the DigiDestined and complete the task by targeting the Destiny Stones. However, Arukenimon and Mummymon's plan of having BlackWarGreymon, one of Arukenimon's creations, destroy the Destiny Stones almost succeeded, but was thwarted at the last minute by Azulongmon. Following this, Oikawa decided it would be easier to weaken the barrier from his own side.
Oikawa instructed Arukenimon to open the Digital Gates in the entire world. During Christmas eve, as the Japanese DigiDestined traveled around the world and helped other DigiDestined close the gates, Arukenimon and Mummymon drove around Tokyo in a truck and abducted twenty children by offering them a chance to be like Ken. The following day, they use the children to force Ken to come with them so that Oikawa could scan his Dark Spore and place copies of it onto the children. After this, Oikawa, Arukenimon and Mummymon ran off to hide in a vacation house in the mountains, but BlackWarGreymon came from the Digital World to destroy them. BlackWarGreymon sensed that Arukenimon and Mummymon were not natural Digimon, which Oikawa confirmed. He also sensed that Oikawa belonged to neither world, due to Myotismon's spirit. Oikawa received unlikely help from the Chosen Children, as they knew they might need him to take the Dark Spores out of the children. As BlackWarGreymon battled WarGreymon and Imperialdramon Fighter Mode Oikawa and his henchmen fled and returned to Oikawa's run-down apartment in Tokyo.
The next day, he confronted Cody Hida, knowing he was a DigiDestined yet unaware he was Hiroki's son. However, as he saw the name Hida on Cody's bag, he realized that this is his best friend's son and fled. That night, he visited Hiroki's grave, plagued by memories and guilt. He didn't let this stop him, however, and went on to absorb the power of the Dark Spore of Noriko Kawada, whose Spore blossomed earlier than expected. He succeeded in this despite the DigiDestined's attempts to stop him. Unexpectedly, Chikara, Cody's grandfather and kendo instructor, happened upon them tried to talk Oikawa down, almost succeeding. However, Myotismon interfered and takes control, using the power he absorbed from the flower, and tries to kill the old man. BlackWarGreymon intercepted the attack and was fatally injured. While everyone was shocked from this, Arukenimon and Mummymon pulled Oikawa away from the scene. BlackWarGreymon, now knowing Oikawa is a puppet like he was, flew into the sky and used the last of his power to seal the Highton View Terrace gate to the Digital World.
After the other Dark Spores had grown strong enough to bloom into flowers, Oikawa gathered all of the children together in Highton View Terrace and used the Dark Spores to try and open a gate to the Digital World. However, he did not know that BlackWarGreymon had sealed the gate with the last of his power. Myotismon sensed this, and made Oikawa open up a portal to an entirely different world, the world of dreams (Although since Myotismon did not witness the sealing of the Digital gate, it could also be considered that the odd dimension selected could have been consciously influenced by Myotismon to regain his form using the power of that world, and should also be noted that the card combination used in Oikawa's computer to open the gate was different from the combination used by the digidestined at Myotismon's gate.). He entered the portal along with the Dark Spore children, his henchmen and the younger DigiDestined, who made it through the portal at the last second. Oikawa was confused when he realized he didn't reach the Digital World, and Myotismon's spirit used the powers of the dream world to remind Oikawa that he had possessed him. Following this, Oikawa fell to his knees in pain and the spirit of Myotismon left him and assumed his appearance before absorbing the Dark Spores of the children to complete his rebirth as MaloMyotismon.
The spirit's departure left Oikawa with barely any life for himself, but he was able to witness the final battle between all of the DigiDestined from around the world against MaloMyotismon. After the battle was over, Cody went back to the world of dreams through a rip that was created in the barrier between the worlds, and tried to bring Oikawa to the Digital World. Thanks to his best friend's son, he finally made it to the Digital world and met the Digimon he had seen so long ago in the video game, Datirimon, who said he was Oikawa's partner Digimon. Oikawa apologized to the DigiDestined for what he had done before using the power of the world of dreams to repair the damaged Digital World by converting his own body into data (shown in the form of a swarm of white/rainbow butterflies), becoming a part of the Digital World itself. This makes Oikawa the only known DigiDestined to die and never recover. 25 years later, his spirit (in the form of butterflies) is still in the digital world.