The League Champion is the final trainer of a Pokémon game's League challenge, fought immediately after the Elite Four, and defeating the Champion enters the player's team into the Hall of Fame and rolls the credits. Champions such as Lance, Cynthia, and Leon anchor their regions as the strongest trainer.
Every main game builds toward this one battle. You clear eight Gyms, cross Victory Road, survive the Elite Four gauntlet, and the Champion is waiting with a full team of six, usually the highest levels in the main story and built with coverage for each other's weaknesses. Blue in Generation I set the template by beating you to the title by minutes. Cynthia in Generation IV became the standard for difficulty, opening with Spiritomb, which had no weaknesses before Generation VI, and closing with a Garchomp that still ends unprepared runs.
Champions are also written as public figures in their regions. Steven is a Devon Corporation heir and stone collector, Diantha is a film star, Leon defends an unbeaten streak in packed stadiums, and Geeta in Generation IX runs the Paldea League as its chairwoman while also serving as Champion. Nemona, your rival in Scarlet and Violet, reaches Champion rank before you and treats your rise as the best thing that ever happened to her.
After the credits, Champions usually remain available for rematches with stronger teams, and several return in later games as postgame opponents. Beating the Champion is the line players use to say a game is finished, even though the postgame is often just starting.
In play"Beat the League Champion on my first try, but Cynthia took me a week of attempts as a kid."