There are seven hidden achievements to be found in-game.

Sonic Unleashed
Use Sonic's All-Star Move to take out Dr. Eggman and show him who's boss!
Feel the Magic
Use Amy's All-Star Move to send Sonic dizzy with love!
Working Man
Use Ryo's All-Star Move to take out Jacky and Akira and show them who's the ultimate martial artist!
Giant Egg
Use Billy's All-Star Move to squash the Crows!
Top Skater
Perform three tricks in one jump with Beat on Tokyo-to - Shibuya Downtown.
The Chariot
Use Zobio and Zobiko to send one hundred Curien Mansion Creatures back to the grave.
Cat Mania
Summon the giant KapuKapu and gobble up Big the Cat for mouse revenge!

For each one you complete you get a badge in your challenges-list (view licence).

Gotta go fast!