How do you unlock cosmo in sonic riders?

You can't unlock Cosmo in Sonic Riders. Cosmo is a character from the Sonic series, but she doesn't appear in Sonic Riders.

Here's why:

* Cosmo is from Sonic X: Cosmo is primarily known from the Sonic X anime, which is a separate continuity from the Sonic Riders games.

* Sonic Riders Character Roster: Sonic Riders features a cast of characters primarily from the main Sonic games, including Sonic, Tails, Knuckles, Amy, Shadow, and others.

If you're looking for a way to play as Cosmo, you would need to explore other Sonic games or media where she appears, such as Sonic X or certain Sonic comics.