Although Amy is a hedgehog like Sonic and Shadow, she still doesn't really have a main part in any of the games. Mainly all she does is chase Sonic around all of the time because of her silly crush on him. Amy and Sonic are still really good friends and they've known each other for a long time, but thats about all she's there for. She's just like another character in the games.
Role in the game:
Amy is only in the movie sequences in the game. She's not actually a playable character except for in the two-player mode. Although near the end of the Light story line, she does get kidnapped my Dr. Eggman, leading into Sonic's level, Crazy Gadget. That's about all she does in the game. She's kind of cool in the two player mode because she can perform moves for less rings but she's actually kind of slow if you're not used to using her.