From Bulbapedia, the community-driven Pokémon encyclopedia.
Marion Town (Japanese: マリオンシティ Marion City) is an anime-only location said to be in Johto featured in Celebi and Joy.
The Pokémon Center in Marion Town
At first, the town was very modern and not embracing of its old traditions, even preparing to demolish a historic Pokémon Center to make way for something new. After Ritchie had gone back in time and changed history, the town also changed into a town that embraced its old traditions, even going as far as restoring the historic Pokémon Center.