Ken's Past...
The first glimpse of Ken in 02 is him as his evil self: the Digimon Emperor. Nobody sympathises him because they don't give any background information. Here is Ken's story:
Ken got his Digivice sometime around the 01 kids did, but he didn't yet know he was a Digidestined, or understand it. He was once a cute, happy normal kid.
Ken once had an older brother named Sam (or Osamu) whom was the real genius in the family. Ken looked up to him, but really, he was jealous and wanted Sam out of the picture so he'd get the attention.
One day, a digivice game out of their computer. Neither of them knew what it was, but Ken felt a strange attraction to it. Ignoring a warning from Sam, Ken touched it and was immediatly sucked into the Digital World.
There, he meets Wormmon for the first time, and Wormmon takes a liking to Ken instantly because of his strong true quality: Kindness. Ken also meets Ryo, a mysterious character. Not much is known about Ryo, just that he has connection to Ken and they were really good friends. I've seen Ryo in pictures with a Veemon, but that's probably not his digimon in the show (if he has one).
Just as things are looking up, there's a turn for the worse. When Sam finds out that Ken played with the digivice, he gets angry and hits Ken for touching his stuff. Ken is hurt, and when he's really mad, he wishes Sam would "just disappear!"
...and then he does :( Sam gets hit by a car, and Ken is effected deep deep down.
Enter Oikawa. Oikawa never fit in anywhere, and it had always been his lifelong dream to visit the Digital World. He's really not a bad person -- he was so desperate to leave the real world that he invited Myotismon to inhabit his body without really knowing what he was getting into. Looking for a way to offset the borders of the Digital World so he could cross over the gate, Oikawa needed somebody to enter the Digital World and build the control spires. After seeing Ken at Sam's funeral, he reconized him as the most vulnerable Digidestined.
After defeating an evil Digimon in the Digital World with Ryo, several dark seeds (or spores) fly through the air. Ken pushes Ryo aside, and one of them is embedded in his neck. Ken screams in agony, and spends the next few weeks in pain. Wormmon took care of him in the Digital World, and soon he recovered and returned to the real world, not quite knowing that what had happened to him and that it would change his life forever.
Then, while playing on Sam's old computer, he gets a mysterious e-mail from somebody who knew about Sam. Oikawa. The e-mail told him that he would need to take over where Sam left off, and when the pressure got to rough, he could turn to the Digital World. Slowly, but surely, Ken began to change. He turned more to his studies and became a soccer star, while distancing himself from any friends he had and his parents. Annoyed with them because he thought himself a higher being.
When Ken returned to the Digital World, he was not the sweet kind kid Wormmon remembered him as. He claimed the Digital World was his, and his alone. Venting out his depression and frustration from his past on the Digital World. The ideas for control spires and dark rings came to him, and he used then while thinking they were a stroke of genius, not knowing that they were causing pain and offsetting the borders of the Digital World. I think you can finish the story. :(
