Keith and his Farfetch'd are Pokémon theifs. Keith would use his Farfetch'd as a distraction and then Keith would swap his victims backpack with a duplicate fills with small rocks.
When Keith was caught he says the reason he steals Pokémon was because he thought his Farfetch'd was too weak to battle Pokémon normally.
After discovering his Farfetch'd is quite strong after a battle with Ash he returns all the backpacks and begins a journey as a Pokémon Trainer.