Keith is a minor antagonist from the episode "So Near, Yet so Farfetch'd." He and his Farfetch'd ran a pretty straightforward theft operation: Farfetch'd would create a distraction while Keith snatched backpacks from unsuspecting trainers, swapping them out with bags full of rocks. Misty, Jessie, and James all fell victim to the scheme, making Misty his fifth known target. Officer Jenny had been tracking him for a while, but he kept moving around before she could pin him down.
The turning point came when Psyduck, one of the Pokémon Keith had stolen from Misty, escaped and led Ash's group straight to him. Once caught, Keith finally revealed his motivation—he genuinely believed his Farfetch'd was too weak to be useful in battle, so he figured stealing stronger Pokémon was the answer. His Farfetch'd quickly proved him wrong by taking down Ash's Bulbasaur in a match. Things went south for Keith after that, though: Misty's Psyduck beat his Farfetch'd using Disable, and when Team Rocket accidentally dumped a load of stolen Voltorb all over him from their balloon, something clicked. He decided stealing wasn't the way forward.
Keith turned over all the stolen Pokémon to their rightful owners, and Officer Jenny cut him a deal—the victims wouldn't press charges if he shaped up. He took the hint and committed to training his Farfetch'd properly instead, focusing on teaching it legitimate catching techniques rather than relying on theft.
Content compiled by AnimeList.moe from publicly available sources.
