From Bulbapedia, the community-driven Pokémon encyclopedia.
Scarlet City (Japanese: カクイシティ Kakui City) is an anime-exclusive city in Johto located at the southern end of Red Rock Isle. The Whirl Cup is held here once every three years in a massive Colosseum, to commemorate Scarlet City's historic connection to the sea. It first appeared at the end of Mantine Overboard!, as Ash and his friends arrived at Red Rock Isle.
Large, white, stone buildings with orange roofs are the predominant architectural style in Scarlet City. Its streets are paved with white flagstone streets. Ash and his friends stayed at the local Pokémon Center, which is perched on a cliff that overlooks the main city.
In Octillery The Outcast, the group met Marcellus, a local and an enthusiastic Remoraid collector. A practice battle against Misty and her Corsola led Marcellus's Remoraid to evolve into an Octillery. Marcellus later escorted them to his local mansion.
Scarlet City's massive Colosseum was the focus in Dueling Heroes and The Perfect Match! as the Whirl Cup took place. Maya and the other Sea Priestesses watched over the event. Ash and Misty advanced far into the competition, though Marcellus failed to pass the preliminaries.