The Digimon Emperor (Digimon Kaizer)
Digimon 02

The Digimon Emperor, or Digimon Kaizer as he is known in Japan, is Ken Ichijouji. Ken had an older brother, Sam. Sam was a genius, and Ken's parents gave him all the attention. Ken felt left out. When Sam died, his parents started pushing Ken to do all the things that Sam might have done. Ken grew resentful and started to lock himself in his room with his computer. While programming, he "created" the Digital World, where he was the evil Digimon Emperor and brought fear to the hearts of all the Digimon. He thought that the Digital World was something that he made up, and when Davis and the gang helped him see that he was wrong, he thought he could just delete all the data and start over. Of course, this wasn't true.
As the Digimon Emperor, Ken invented the Dark Rings and Dark Spirals, allowing him to control Digimon with the help of Control Spires (tall black towers). He wanted to take over the whole Digital World (sound familiar?). He also thought that he was perfect, and only perfect children should be in the Digital World - so he decided he wanted to destroy Davis and the other Digidestined...
More information: Ken's bio