From Bulbapedia, the community-driven Pokémon encyclopedia.

Fennel Valley is an anime-exclusive location in Kanto that is first visited in the episode Overjoyed!. It is visited by Ash and his friends because it is the second location of the Battle Pyramid which Ash has to battle to obtain the Brave Symbol. He loses to Brandon but keeps training in the valley until the episode Gathering the Gang of Four where Ash challenges Brandon again and finally wins his last Symbol to become the Battle Frontier Champion. There is a very luxurious Pokémon Center with a fitness studio, hot springs, battle fields and even a Contest Hall.