Kai, along with Maury and Allegra, challenged Ash and his friends to a battle; specifically, he went up against Dawn and her Piplup, which began as a puff-contest (befitting their species' nature).
Kai was later invited by Baron Alberto to help fight off Darkrai.
During the end credits, he was seen participating in the Alamos Town Pokémon Contest, where he defeated Maury in the Battle Stage, and was likely defeated by Allegra in the finals as he was on the stage with her when she received the Ribbon. Bulbapedia