Westside Island is an island that Sonic the Hedgehog visits after saving South Island from Dr. Robotnik, and where he first met Miles "Tails" Prower. After Sonic the Hedgehog 2, many zones of the island make reappearances in later games, even if Westside Island itself is not mentioned.
Sonic the Hedgehog 2
Westside Island first appears as the main setting in Sonic the Hedgehog 2. Sonic the Hedgehog was looking for new adventures following the events of Sonic the Hedgehog when he discovered the island and recalled its legend. A long time ago, the people of Westside Island used a mysterious stone to advance their civilization and achieve prosperity. However, when those people tried to use the stone for the wrong reasons, their prosperity vanished overnight. The Gods were said to have sealed away the stone within the island's depths. Sonic landed his plane, the Tornado, on the island to check it out and relax, not paying attention to the fact that Robotnik was following him and suspiciously landed on the other side of the island.
A few days later, Sonic noticed a shy orange fox following him. Sonic noticed that the peculiar fox managed to barely keep up with him, but decided he was harmless and became friends. The fox's name was Miles Prower, though he was known as "Tails" due to his two special tails. It turned out that Tails had heard of Sonic since he was a cub, and wanted to become cool like him. During this time, Tails gained the respect of the island's animals by being able to imitate Sonic's famous moves. He learned he could closely replicate Sonic's trademark speed by twirling his tails like a propeller along the ground, could roll into a Spin Attack, and even perform a Spin Dash.
One early afternoon, Tails discovered Sonic's plane at the beach. Being a fan of machinery, Tails admired it while Sonic was taking a nap under its wing. Suddenly, explosions shook the island and Tails took cover underneath it as he saw robots violently excavating the island. It turned out that Dr. Robotnik detected the presence of all Chaos Emeralds after following Sonic - including the lost seventh one spoken of in the island's legend. He took the opportunity to kidnap the island's animals to rebuild his Badnik forces and finish his ultimate weapon, the Death Egg. Robotnik now ruled the factories, refineries and cities. He's grabbed control of everything — except the seven Chaos Emeralds. Sonic went off into the forest to set things right, and Tails followed.
Sonic and Tails journeys through all zones of the game to chase down Robotnik. Eventually, Robotnik makes his retreat from the island. Sonic and Tails take to the skies of Sky Chase Zone as they pursue Robotnik's flying ship, Wing Fortress. The Tornado is shot down, taking its pilot with it. The player character (Sonic by default) takes on the fortress only to find Robotnik escaping again. However, Tails comes back with a hasty rocket engine repair. Sonic jumps off the Tornado and onto Robotnik's escape vehicle, managing to enter the newly-created Death Egg Zone. Sonic manages to defeat the final line of defense: Mecha Sonic and the Death Egg Robot. Just a moment later, the Death Egg explodes as the bright light could be seen above the Westside Island. Tails finds Sonic with the Tornado in the middle of the sky and thus the island was free.
- Emerald Hill Zone - A bumpy, tropical peninsula plain riddled with abandoned emerald mines.
- Chemical Plant Zone - A factory built to manufacture industrial chemicals (possibly to supply Metropolis).
- Aquatic Ruin Zone - The half-submerged ruins of an ancient city.
- Casino Night Zone - A casino full of jackpot machines and similar that supplies Westside island with tourism money.
- Hill Top Zone - A mountainous region of tall "block mountains" and lava- flooded caves.
- Mystic Cave Zone - An extensive system of crystal mines full of bridges, spikes, and other obstacles.
- Oil Ocean Zone - A massive valley flooded by the spillage from various oil rigs (which probably supply Metropolis).
- Metropolis Zone - A huge, industrial city made entirely of pipes, mechanical contraptions, and huge factories. It was briefly used by Dr. Robotnik as a badnik manufacturing plant.
And above the island:
- Sky Chase Zone, where flying badniks patrol the island's airspace.
- Wing Fortress Zone, the Doctor's aerial fortress.
And even higher above:
- Death Egg Zone, Dr. Robotnik's own personal doomsday moonlet.
Sonic Drift 2
Many zones of Westside Island are seen as multiple race tracks in Sonic Drift 2, although once again the island itself is not mentioned at any form. Many of these included Emerald Hill Zone, Hill Top Zone, Casino Night Zone and Mystic Cave Zone. There are also some original tracks as well, that would take place of the island such as Balloon Panic, which would possibly taken place of Emerald Hill Zone.
It seems that Tails' Skypatrol might take place on Westside Island (although this was never confirmed), as many its zones seem to have parallels. The Training Area looks like Emerald Hill Zone, RailCanyon Area is similar to Hill Top Zone, RuinWood Area looks like Mystic Cave Zone, and DarkCastle Area looks closely related to Casino Night Zone. In the game's storyline, Witchcart tries to claim the entire island for her own and threatens to turn any dissenters into a crystal.
Sonic the Fighters
After the first appearance, some of Westside Island's zones later appear as fighting arenas in Sonic the Fighters. One of them is Casino Night, where the player's opponent is Fang the Sniper. Also several other arenas looks like familiar to the other zones of Sonic the Hedgehog 2. For example, Dynamite Plant is mixed of Chemical Plant Zone and Oil Ocean Zone, as well Giant Wing bears resemble of Wing Fortress Zone.
At the intro demo of the game, it also seems that Tails's workshop might actually located on Westside Island.
Sonic Chronicles: The Dark Brotherhood
Westside Island (although not being mentioned by its name) might appear as one of the islands in Sonic Chronicles: The Dark Brotherhood, due to ruins of Metropolis being there. This also would confirm Blue Ridge Zone being as part of the island.
Appearances in Other Media
In the Archie Comics, Westside Island is a location on Mobius. Presumably, it did not initially exist on Mobius until after the Prime Zone's timeline and reality was rewritten by the Super Genesis Wave. It is the location of Mobotropolis, Knothole Village and the Wood Zone.