Koichi Kimura is a fictional character in the anime series Digimon Frontier. Koichi is the final member to join the team. He was chosen to wield the Spirits of Darkness that hold AncientSphinxmon's power.

He is Koji Minamoto's twin brother. Their parents divorced when they were very young, so they hardly knew about each other. Koichi learned about Koji when his grandmother told him on her deathbed. He ran after Koji, who was on his way to the Digital World, to try and catch up with him but he fell down a flight of stairs and ended up in a coma as a result. His mind was taken into the Digital World by Cherubimon, who saw his sadness. He erased Koichi's memories and gave him the Human Spirit of Darkness to give him a semi-physical form as Duskmon until he was freed from Cherubimon's control by Aldamon and Beowolfmon. After this, he wielded the purified Spirits of Darkness and fought alongside his brother and the other DigiDestined.

Later on, Koichi began to realize that, in reality, his mind was not truly in the Digital World. On multiple occasions, the DigiDestined were seriously injured, revealing their Fractal Code (Digi-Code in Japan). However, each time Koichi is far less hurt and reveals no Fractal Code. Crusadermon informed him that he was only a ghost within the Digital World and this troubled him towards the end of the series. As the final battles with Lucemon ensued, Koichi sacrificed himself to save the other DigiDestined, giving his spirits of darkness to his brother, Koji. The powers of light and darkness begin to react within Koji's body, helping trigger Koji and Takuya's final evolution into Susanoomon. In the final episode it is revealed by Löwemon that Koichi is still alive but is badly hurt. As Koji burst into the emergency room, he realized he was too late to save his brother. But in the end, the other DigiDestineds' D-Tector Digivices were able to revive him after his death just moments earlier.

Source: Wikipedia.