At the begining of the game the 2nd world. Looks like a Sonic head with Green eyes. That's the only planet i know that lets you play 2 player. There might be more but im not that far yet.
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In Sonic the Hedgehog(2006), You can just play as him in multiplayer mode or complete certain levels in campain mode.
No It does feature Sonic 2 and 3, but there is no way to make it multiplayer, and they removed the multiplayer (as well as options) from the games.
You can only do 2 player in the Sonic Simulator Mode.
You have to unloock diferrent Cups and play Multiplayer to unlock character costumes
The light speed dash in not in sonic colors