Leon is
The Galar Region Champion and brother to
Hop, the same who has never lost a single Pokemon Battle in his entire life. Your trainer will first meet Leon when he gifts both you and Hop with your
Starter Pokemon, and you will run into him on a number of instances as the Main Story progresses. Usually, you will only run into Leon when
Chairman Rose is experimenting with the Concept of
Dynamax, where Leon will have to either stop a Pokemon that has Dynamax or investigate a loud bang from what
Rose is doing. In each of these encounters, Leon will simply tell your trainer to focus on completing the Gym Challenge.
Eventually, after winning the
Champion Cup Semi-Finals, it will be up to your trainer and
Hop to ascend
Rose Tower in order to find
Leon with
Chairman Rose. As Rose decides to executes his plan of awakening
Eternatus in hopes of preventing the energy crisis from occurring in a thousand years, Leon will be thrown into action once again directly before your Championship Match with him. Leon will ultimately stop Eternatus in its normal form, but he will fail to capture it. Instead, it will be up to your trainer to do so.
Once
Eternatus has been stopped and captured, you will finally have your Championship Match with
Leon. Defeating him will see your trainer become the next
Galar Region Champion and conclude the Main Story. From there, you won't see Leon again until you reach a certain streak at the
Battle Tower. When you do, he will battle you in the Battle Tower Format. Should you defeat Leon just once at the Battle Tower, he can be found from there on out as a possible participant if you rematch the Champion Cup (having one of the strongest Pokemon Teams in the entire Galar Region).
Later on after defeating the
Crown Tundra DLC,
Leon will be a participant at that
Galarian Star Tournament. Furthermore, after defeating both the
Isle of Armor and Crown Tundra DLC Packages, Leon's Champion Cup Rematch
Pokemon Team will see a bump in their levels.