Leon originally started out as a Trainer at the age of 10 with his partner Charizard when he entered the Gym Challenge after he was endorsed by Rose, who saw talent in him. He proceeded to win his first-ever Gym Challenge and become the Champion of the Galar League, without losing a single battle. Since then, he and his partner have remained undefeated in every league and exhibition they participated in. He has been referred to as "the unbeatable Champion" and "the greatest Trainer in all of Galar." He is considered a hero to many Trainers in Galar. However, he is also known for his poor sense of directions. His catchphrase is "Let's have a champion time!". According to Mustard, Leon was once his best student at the Master Dojo, but he was never able to finish his training there, as he never made it to the Towers of Two Fists due to his horrible sense of direction.
Leon first appears in the games' introduction movie, where he is introduced as Galar's undefeated Champion by Rose at Wyndon Stadium. He exchanges greetings with his opponent Raihan, who is challenging him with Duraludon, before Gigantamaxing his Charizard. Their subsequent battle is not shown on-screen, but Leon defeats Raihan to retain his title.
Later, Leon travels via train to Wedgehurst, where he entertains a crowd outside Wedgehurst Station with Charizard until Hop and the player meet him. Together, the trio return to Hop and Leon's home in Postwick. Once there, Leon reveals the gifts he brought for Hop and the player — the starter PokémonGrookey, Scorbunny and Sobble. He allows the player and Hop to choose which of the three Pokémon they want, then adds the remaining Pokémon to his own party. Leon then watches the battle between the player and Hop before joining in a barbecue dinner he and Hop's mother hosts at their home. Hop also asks Leon to endorse him and the player for the Gym Challenge, but he is initially hesitant to do so because of their inexperience. Later, when Hop and the player run into the Slumbering Weald to save a Wooloo, Leon eventually runs in and finds them unconscious, and manages to save the Wooloo.
Leon meets Hop and the player again at Professor Magnolia's Laboratory, and after Magnolia encourages him to endorse Hop and the player, he asks them to engage in a battle. After the player defeats Hop in a formal battle, Leon decides to write letters of endorsement ensuring they are eligible to participate in the Gym Challenge.
Leon appears throughout the player's journey. During the Champion Cup, Leon invites the player and Hop to a dinner after the semifinals, but he ends up late due to having a heated, passionate conversation with Rose about the future of Galar atop Rose Tower. Leon argues that he must fulfill his duty as a Champion primarily before dealing with the distant future. Hop and the player manage to find him after dealing with several Macro Cosmos employees and Oleana, and manage to find him. Later, when the player wins the finals, before they can have a final match with Leon, Rose interrupts the League to announce that he has summoned Eternatus and intends to solve the future energy crisis by bringing about the Darkest Day. Leon rushes to Hammerlocke Stadium to deal with the situation, and manages to hold off Eternatus until Hop and the player arrive, but is unable to catch Eternatus and is left to take the resulting energy blast from Eternatus breaking out of his Poké Ball. He spends the next few days recovering from his injuries after the player, Hop, Zacian, and Zamazenta successfully stop Eternatus. The player and Leon finally have their grand showdown, which invites an incredibly large amount of fans across Galar. The player prevails over Leon, and becomes the new Champion.
After the events of main story, he gained ownership of Rose Tower and transformed it into the Battle Tower. His reason for this was to fulfill a lifelong dream he had while he was still Champion, where Galarian Trainers can become the strongest in the world. Another reason behind his ownership of the Battle Tower was so that he can never get lost inside it. Some say Leon has become more lively than when he was Champion and is currently having a hard time looking for a new catchphrase to use.
Leon also becomes the new Chairman of the Galar League. After the player has completed the events of The Isle of Armor and The Crown Tundra, Leon calls the player and invites them, along with Hop, Avery or Klara, Peony, and all of the Gym Leaders to participate in the Galarian Star Tournament, an event he created for the Galar League that is focused on Multi Battles. After the first Galarian Star Tournament, Leon announces that there will be many more Star Tournaments held in the future. It is later revealed that he received the funding for the tournament from Sordward and Shielbert as their way of atoning for the crimes they committed in postgame.
In the English versions of the games, Hop refers to Leon by the nickname Lee. In all other languages, he is merely referred to as "older brother" or "brother" (Japanese: アニキbig bro).
"Leon is known far and wide for taking part in the Gym Challenge at the tender age of 10 years old—and even more so for claiming victory in the Champion Cup on his first attempt. Ever since that victory, he and his partner Charizard have remained unbeaten in both League and exhibition matches. He is called "the unbeatable Champion" and is a hero to many Trainers. He has but one weakness: a dismal sense of direction. But don't worry—wherever he ends up, he makes sure everyone has a champion time!"
Rare League Card
"Leon's dream, even when he was still the Champion, was to make Galarian Trainers the strongest Trainers in the world. In order to fulfill that dream, he has rebuilt Rose Tower into the Battle Tower, a facility dedicated to Pokémon battles. People say that he seems even more lively now than he was as Champion. Leon has also commented that another reason he likes the tower so much is that it's impossible to get lost inside. Leon's biggest problem now is coming up with a new signature catchphrase to replace "Let's have a champion time!""
In the Battle Tower, Leon's team is randomly selected each time, but with Charizard guaranteed in the last slot (which he may still switch into at any time). He will Gigantamax his Charizard at the first opportunity.