He was a brash young Trainer who wanted a Cyndaquil, and ended up competing with Ash to catch one in a forest. Even though Ash caught one first, Koji pressured Ash into a battle, with the prize being the Cyndaquil Ash already caught. Against Brock's better judgment, Ash accepted the challenge and had Cyndaquil battle Koji's Sandslash. During the battle, Koji was shown to be a confident Trainer with a powerful Pokémon. He is not a kind Trainer, and pressured his Sandslash to continue the battle even though it was exhausted.
Despite having a disadvantage, Cyndaquil was able to use its speed and Tackle attack to defeat the Sandslash. Koji then decided to try to steal Cyndaquil anyway in a net, only to get badly burned when it used a Flamethrower at him, causing him to run away. As he fled, Brock yelled after him that he should train himself before training any more Pokémon.
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Sandslash is Koji's only known Pokémon. He used it in his attempt to catch a wild Cyndaquil that he found: first to try and stop Ash from catching the Cyndaquil, and later to battle the Cyndaquil once Ash had caught it. In the battle, Sandslash was shown to be very agile and powerful, but it was ultimately defeated by Cyndaquil's Tackle attack after getting stuck in a tree. During the battle, Brock noted that Sandslash's Slash attack had "amazing destructive power". Koji mentioned that Sandslash's best move was Fury Swipes.