HISTORY
Kouichi was born as a set of twins— not that he knew that while growing up, because his parents divorced not long after and each took one child. His mother reverted to her maiden name, so he grew up with the family name "Kimura" instead of his father and brother's name, "Minamoto." His mother never told Kouichi about his younger twin, Kouji, nor much about his father. Not that he minded, because his mother was a kind person who worked hard at her job so she could provide a good life for her son. When his grandmother was on her death bed, though, she told Kouichi about the sibling he never knew.
For his part, Kouichi decided he wanted to know what kind of person this "Kouji" was, so he tracked him down and followed him. He didn't have the courage to approach Kouji, though, especially after seeing the peaceful family life he had with their father and his (the father's) new wife. He ended up following Kouji a few more times, eventually ending up at a train station where Kouji went to the Digital World on a Trailmon— and Kouichi fell down a flight of stairs.
Kouichi woke up in the Digital World and soon encountered Cherubimon, who saw the darkness in Kouichi's heart, the sadness, jealousy, and anger about Kouji. Cherubimon gave Kouichi power, the corrupted Spirit of Darkness, and Kouichi became Duskmon, with no memories of who he was and nothing but the darkness remaining in his heart. As Duskmon, he joined forces with the other Digimon working with Cherubimon to eat all the data in the Digital World. Not that Duskmon did much of that, prefering to sit outside and brood.
Duskmon eventually does fight the Chosen Children in their digimon forms, though, and very nearly kills them all. When one of the digimon he's fighting turns into Kouji, it shakes something deep inside him, and he flees. He eventually faces Kouji again, nearly killing him under Cherubimon's machinations when the angel digimon helping the other kids tries to restore Kouichi's memories. He flees again, gets manipulated again, and fights Kouji again— but this time, he reveals who he is to Kouji. Duskmon gets defeated finally, and his spirits are purified.
Although he feels guilty for nearly killing everyone, he's soon absorbed into their group. Fighting a shadow of Cherubimon ends up with him getting the purified versions of the Spirits of Darkness, and he's given a digivice and new forms to fight in— Löwemon instead of Duskmon, and KaiserLeomon instead of the evil Velgmon he'd previously had. Together wtih his new friends, they defeat Cherubimon at last... only to discover that he wasn't the only threat to the Digital World.
With the revival of Lucemon (an overpowered angel digimon) imminent, the team vow to stop him. But Lucemon has sent out his two minions, the Royal Knights, and nothing in their arsenal is enough to seriously stop them. No matter how hard they try, the Royal Knights end up taking the code of the whole Digital World. Lucemon has been revived, and the kids escape to the moon. Meanwhile, Kouichi has made a startling discovery: He might not have actually survived his trip to the Digital World. . .