The park warden is a minor character from "One Trick Phony!" who runs the Battle Park. He's mostly memorable for handing Team Rocket an endless supply of dirty Poké Balls to clean—and laughing at their complaints about it. When he needed to grab supplies, he left them in charge of the whole facility, which in retrospect was probably not his best call.
Things went downhill fast. Team Rocket grabbed the cleaned Poké Balls along with Ash's Pikachu and took off, prompting the warden to chase after them yelling that those Poké Balls were park property. The situation spiraled when the rental Pokémon Team Rocket had deployed—Venusaur, Charizard, and Blastoise—decided they didn't feel like listening to anyone outside the park's boundaries. Ash's Totodile and Misty's Politoed brought down their hot air balloon, and the rental team turned on them instead, sending the trio flying.
Content compiled by AnimeList.moe from publicly available sources.