There are various of ways to unlock the characters throughout the game. Don't fret if you can't unlock them all, it's relatively easy. Here's how you can unlock other characters, including Super Sonic. Although, there are no other ways to unlock Tails, but Sonic. Everything explained here are ways to unlock characters at earlier stages. Don't worry if you missed Big if you are using Sonic. We all made a mistake or forgotten. Anyways, you can always acquire Big by using Tails. Which is a guaranteed Character Unlock.
If you completed E-102 Gamma's story, you will know that he will no longer take part in any other Sonic the Hedgehog games, but Sonic Battle. You can still play as him as much as you like, but there is no guarantee he will appear in other games. That is all.
|Tails||Sonic: Complete the first level of the game.|
|Knuckles||Sonic or Tails: Defeat Knuckles in a battle and defeat Chaos 4.|
|Amy||Sonic: Wonder around at Casino Plaza. Tails: Defeat E-102 Gamma in a battle.|
|Big||Sonic: Talk to him at the train station. Tails: Get Froggie in the desert sand stage.|
|E-102 Gamma||Sonic or Tails: Defeat E-102 Gamma in a battle. Amy: After the cutscene with E-102 Gamma. Knuckles: After the cutscene where Knuckles first spotted E-102 Gamma and found a way to Eggman's ship.|
|Super Sonic||Complete all character's story.|