This is about the anime characters. For the monster itself, see Kuro.
The Kuros first tangle with Genki and his team when they try to intercept a man who escaped from Pixie's labor camps in Episode 9. Ed is able to make it all the way across Heaven's Canyon, but Captain Kuro decides to use the escape to his advantage. He waits until Ed is alone, and then makes him a deal: if he rigs the iron bird that the Searchers are going to use to cross the canyon, causing it to explode, the Kuros will let him go free.
Ed almost goes through with the plan, until the Searchers take off. He changes his mind at the last minute and shouts a warning, and the Kuros attack. During the ensuing battle, Ed and Captain Kuro both fall off of the craft. Ed is able to catch the side of the canyon, but the captain plummets to his death.
Losing their captain doesn't seem to deter them, because the Kuros later return with a new captain in Episode 31. Genki and the others are at the ruins of an amusement park, and are forced to retreat when the Searchers receive backup from Pixie.