Yu-Gi-Oh! episode 63: "Playing with a Parasite, Part 1"
Haga promised the boy a rare card if he could sneak the card "Parasite Paracide" into Jonouchi's Deck. After trailing Jonouchi for over an hour, the boy finally got a chance when Jonouchi entered Beef Buster and took off his Duel Disk at the manager's request. He immediately snatched the Duel Disk and ran out of the restaurant, with Jonouchi in hot pursuit.
Eventually, Jonouchi caught up to the boy, but not before he successfully infected Jonouchi's Deck, as per Haga's instructions. The boy then spun a story to Jonouchi about how he was a Battle City contestant that was defeated in his first Duel and had his own Duel Disk and Deck stolen by his opponent. Jonouchi fell for the story and let the boy go, telling him to find the thief so he himself could Duel him and win the boy's Duel Disk and Deck back.
The boy eventually met up with Haga and told him that the plan was a success. Haga gave him a common "Gokibore" card as compensation, making him protest that Haga had promised him a rare card. In response, Haga sprayed a can of insecticide in the boy's eyes.
The boy's motivations and impact on the plot are more or less the same in the anime. Instead of simply stealing Joey's Duel Disk, he claimed to be a fan collecting autographs from skilled Duelists. He then asked to hold Joey's Duel Disk and promptly ran away with it when Joey consented.
He was caught by Tristan instead of Joey himself and explained that he took the Duel Disk to become a duelist. During his interactions with Weevil, since the boy was unsatisfied with the card he received for his efforts, Weevil instead attacked him with a spider-shaped gun that shot silly string.