He first appeared in The Visitor. After meeting Fabia and being asked by her if he got her message, he lied saying he did to get her attention. She then challenged him to a brawl, to know if he really did get it. Because Koji underestimated her, he lost.
He reappears in episode 22, trying to get into Bakugan Interspace. Julie stops him and tells him about the Neathian-Gundalian war. He leaves, thinking her story is too farfetched and is approached by Stoica who convinces him to go to Gundalia where he is quickly brainwashed by Kazarina. He battles Fabia and Dan with the Mobile Assault Impalaton, and nearly wins, but loses when Drago uses his own Mobile Assault, Raytheus. He is returned to Earth and wakes up in Bakugan Interspace, having no memory of what transpired. He tells Julie about what happened and vaguely remembers Dan's voice telling him "I won't give up on you!"
He appeared at the end of episode 39, and started to battle Dan and said that he had been waiting to battle him, but then Dan said that he already did, but then Drago told him that Koji has no recollection of their previous battle.
His voice in episode 3 of Bakugan: Gundalian Invaders sounded a lot like Baron Leltoy, presumably having Cameron Kennedy voice him for that occasion. Later on, it was Alex House, the voice of Ace Grit, who did the voicing of Koji.