Lance is a character first introduced in the Kanto-based Pokémon games. He specializes in the Dragon type. He is the older cousin of Claire.
Game Canon:
Lance first appears in the Kanto-based Pokemon games (Red, Green, Blue, Yellow, FireRed, LeafGreen, Let's Go Pikachu, and Let's Go Eevee) as one of the members of the Elite Four. He was originally the Champion of the Indigo League, but due to blue-game or trace-game beating him, he's been bumped down to the fourth member of the Elite Four by the time the player (Red_Anime, leaf-game, chase-game, or Elaine_Game, gets to him.
Two years later, in the Johto based games (Gold, Silver, Crystal, HeartGold, and SoulSilver), he has gotten his Champion position back after Red beat Blue and left to train at Mt. Silver.
In Black 2 and White 2, he travels to Unova to fight under the title of Johto Champion in the Pokémon World Tournament.
In the Alola based games (Sun, Moon, UltraSun, and UltraMoon), it's revealed Professor Kukui faced Lance at one point as Champion and lost.
Pokémon the Series:
Lance is a master Dragon-type Trainer and member of the Kanto Elite Four.
In the finals of the Pokémon World Championships, the tournament that determines the world's best at Pokémon battling,
Leon battled against
Lance, the winner of the Elite Four Cup in the Kanto region. In the one-on-one final, the two Daimaxed their Pokémon but Leon's Charizard came out victorious.
Leon was then crowned world champion.