Osamu "Sam" Ichijouji
Sam was Ken's older brother. He was a genius and Ken really looked up to him.
Everyone liked Sam best and Ken felt left out. Sometimes he wished Sam would
just disappear, and he did when he died in an accident.
One day, a Digivice came out of Sam's computer and Sam put it away, telling Ken not to touch it. Ken just wanted to play with it, but it sucked him into the Digital World. He loved it there but Sam caught him when he came back and hit him. Ken got really sad, and he felt like it was his fault when Sam died because he wanted him to disappear.
After Sam died, Ken was told that he had to take on two destinies. This implies that Sam was supposed to be the Digimon Emperor - which would explain the hairstyle Ken uses as the Digimon Emperor (it's exactly the same as Sam's), and the dark glasses.