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.