Befriended: "I did lose this hardcore battle, but... now that we traded blows, we're already friends, right?!"
Gachin-kozo debuted in episode 138, where he inspirited Katie. Thinking that Gachin-kozo was a challenge, Nate decided to challenge him in the schoolyard. He summons Kyubi, Venoct, Robonyan F, Snartle, Lie-in Heart, and Shogunyan to attack Gachin-kozo. Nate summoned the almighty Lord Enma to finish him off, presumably destroying him.
"Gachin-kozō" is a combination of gachinko (ガチンコ, "hardcore/earnest competition") and kozō (小僧, "youngster").