Rosewood Private School student Koji (in the Japanese version, he goes to Seidou Gakuen (青銅学院), meaning Bronze Institute) is the mollycoddled and rather snooty son of an extremely rich family. Something of an ace medafighter, Kouji and his medarot Sumilidon are legendary for never losing a robattle. Kouji and Ikki are rivals, but manage to co-exist together because of their crush on Karin. Kouji always trains hard with his medarot, Sumilidon, trying to master the technique known as the Shadow Sword, taught to them by Space Medafighter X. He thinks of himself as Karin's boyfriend. He's afraid of any insect, due to collecting a mantis egg sack as a child and forgetting about it, which lead to the egg hatching into a number of mantis that frighten him. He was even frightened of the Medabot Spidar, calling it a big bug, although he seems to either not care of Metabee's appearance of a Hercules Beetle, or doesn't see the resemblance. He, along with Ikki and Space Medafighter X, participate in the World Robattle Tournament as Team Japan.