The Sevii Islands are a group of islands located south of the Kanto region. They are not to be confused with the Orange Islands, another island group near Kanto, or the Blue Islands near Hoenn. The Sevii Islands are accessible in Pokémon FireRed and LeafGreen after the player defeats Blaine. They serve the purpose of a major side-quest involving Bill and is also where the player can catch Pokémon from Pokémon Gold and Silver. There are nine islands in all, and each of them is accessible once the player does something special. The islanders are ferried between the islands by the Seagallop Hi-Speed 6.
According to an old lady on Quest Island, the islands were named such because they were formed in seven days. The name is probably a portmanteau of "seven" and "VII" (Roman numeral for seven). It is also possible that the islands may have a reference to the legendary Pokémon Ho-Oh since there are 7 main islands and a rainbow has 7 different colors, relating to Ho-Oh being the Rainbow Pokémon. This is also possible how Ho-Oh roosts on Navel Rock but then roosts in Johto on the Tin Tower (Later named Bell Tower in Generation IV).
In the Games
The Sevii Islands only appear in-game in a sidequest available after defeating Blaine in Pokémon FireRed and LeafGreen. Bill asks for the player's help, and the player then has to solve problems on islands One , Two, and Three, where a scientist needs help, a girl has disapeared, and bikers are invading.
In the Anime
The Sevii Islands only appear in an episode of the Pokémon Chronicles series, as the setting of the episode.