The Sevii Islands are the one and only addition to Kanto Region found in Pokemon FireRed compared to the 1st Generation Pokemon Games. In total, there are nine islands apart of the Sevii Islands; however, only seven are accessible without a Pokemon Event Exclusive Item. Players will be able to explore the first three of Sevii Islands after obtaining the first seven Pokemon Gym Badges, with Bill, the PC Box Creator, being the one to introduce the idea of doing so. As players explore the first three Sevii Islands during this trip, they will need to complete a mini story revolving around saving Lostelle on Three Island.
After defeating the Main Story and obtaining the National Pokedex, players can return to One Island in order to defeat Team Rocket and find the Ruby deep within Mt. Ember. After returning the Ruby to Celio, players will be able to explore the next four Sevii Islands. This is officially the Post Game Story found in Pokemon FireRed, which sees players having to find the Sapphire in order for Celio to complete his machine as they defeat Team Rocket once and for all.
The final two Sevii Islands are Naval Rock and Birth Island; however, they can only be explored by obtaining the MysticTicket or AuroraTicket. These two items were Pokemon Event Exclusives way back when, but they can still be obtained by using Cheating Code Devices such as a Code Breaker or Action Replay. The Legendary Pokemon Lugia and Ho-Oh can be found on Naval Rock, while the Legendary Pokemon Deoxys can be found on Birth Island.
Now that you know all about the Sevii Islands, Click Here to learn about the rest of the Kanto Region!