Cosmo is a character from the Sonic series, specifically from the Sonic X anime and the Sonic series comic books. She is not part of the original Sonic Riders game.
The playable characters in Sonic Riders 1 are:
* Sonic the Hedgehog
* Tails
* Knuckles
* Amy Rose
* Shadow the Hedgehog
* Rouge the Bat
* Jet the Hawk
* Wave the Swallow
* Storm the Albatross
* E-123 Omega (unlockable)
Let me know if you have any other questions about Sonic Riders or the Sonic series!