The Sinnoh League is a Pokémon competition held in the Sinnoh region, where trainers battle to prove their skills. It consists of various stages, including the Pokémon League Challenge, where trainers compete against Gym Leaders to earn badges. Once they collect all eight badges, they can enter the Sinnoh League tournament. The tournament culminates in a battle against the Elite Four and the Champion, who is the top trainer in the region. Notable characters in the Sinnoh League include Ash Ketchum, who aims to become a Pokémon Master, and Cynthia, the Champion known for her powerful Pokémon.