SONIC FREE RIDERS

To unlock all characters you must play through the world grand Prix, once you've completed both team Heroes and Team Babylon stories you will unlock Team Dark and Team Rose, once you've completed those the final story will unlock, when completed you will have a full character roster, enjoy.

To get the full roster of playable characters, you need to complete all the stories from the World Grand Prix, aka, the Story Mode, you start with Team Heroes and Team Babylon, once you beat their stories, you'll unlock Team Rose and Team Dark, after beating their stories you get the final story where you'll unlock the last few characters after beating it, giving you the achievement, the characters are as follows:

Amy Rose, Cream the Rabbit, Vector the Crocodile, Shadow the Hedgehog, Rouge the Bat and E-10000G - Beat Team Heroes and Team Babylon's stories

Blaze the Cat and Silver the Hedgehog - Beat Team Rose and Team Dark's stories
(Have fun playing as the best character in the franchise: Blaze)

Dr. Ivo "Eggman" Robotnik and Metal Sonic - Beat the whole World Grand Prix

Note: If you win a race with Sonic, Jet and E-10000G in every single track, you get Avatar Items, those being T-Shirts of Sonic, Jet and E-10000G, if you feel like exercising a bit more by playing Kinect, then maybe getting these Avatar Items will be a pleasant (or painful) exercise for your unfit body, good luck! (You'll need it)