Cosmo is a plant-like animal that comes from another planet. She tells Sonic and company about the new enemy "The Metarex." Their new mission is to find the Chaos Emeralds scattered around the galaxy before The Metarex do or else they'll take over the whole galaxy!
At first Knuckles and Cosmo seem to like each other, but then in episode 62 Tails saves Cosmo's life, and the two start to develop a crush on each other. Tails will do anything to protect her... no matter what, even if it means to taking a serious beat down!
Like Cream in season 1, Cosmo is defenseless and must rely on someone. Take episode 73, for example. Shadow is trying to kill Cosmo because she is a spy for the enemy. Chris is badly wounded, Sonic is nearly killed, Knuckles gets K.O.ed, and Tails is taking a lot of damage! Cosmo is involved in the last fight toward The Metarex. In order to save everybody, Cosmo sacrifices herself (since Sonic doesn't want Shadow to be gone AGAIN!) to weaken them leaving Tails free to destroy them. Of course Tails made a promise to Cosmo and if he fired, it would be broken and she would be killed. Eggman and Cosmo tell him to do it, and he fires leaving him in tears of sorrow.