Location from pokemon-adventures-frlg by
A remote archipelago south of Kanto -- seven inhabited islands plus two hidden ones, invaded by Team Rocket and turned into a battleground for the fate of the Pokedex holders.
The Sevii Islands are where this arc lives and breathes. Red and Blue arrive on One Island by ferry, expecting a simple errand. What they find is a chain of communities under siege -- the Three Beasts have spread across the archipelago, attacking each island systematically. The islands served multiple strategic purposes for Team Rocket: the Rocket Warehouse on Five Island stored the Ruby and Sapphire gems, Trainer Tower on Seven Island housed the AI computer R and held hostages, and Birth Island (hidden from most maps) was where Deoxys retreated to heal. For the Pokedex holders, the islands became a crucible. They trained under Ultima on Two Island, fought across Five, Six, and Seven Islands, and launched their final assault from the archipelago toward the airship above Viridian City.
A chain of tropical and subtropical islands scattered across blue ocean, each with its own character. Volcanic peaks on One Island, lush forests on Three Island, frozen caves on Four Island, ancient ruins on Seven Island. The sea between them is vast and isolating -- these islands were not meant to be found easily.
Also known as: Sevii Islands, the islands