Cosmo is a female child of a plant-like alien species whose homeworld was destroyed by the villainous Metarex. She is an ally of Sonic the Hedgehog and his friends, and it is widely accepted that her primary purpose in the series is that of a love interest for Tails, the two characters being of the same age and displaying an overt fondness for one another.
In Sonic XEdit
Cosmo first appeared in episode 53 of Sonic X. Her former home world was destroyed by the Metarex before she was born, and she was instead raised on a space colony. She lived peacefully there until the Metarex came and began to destroy her new home, including her family (including her mother, Hertia). As Cosmo was escaping, the Metarex planted a tracking device on her. Cosmo also had a device inside her brain which, when activated, allowed the Metarex to hear and see everything that she did.
After the attack, Cosmo escaped into a ship and arrived on Sonic's planet. Tails, Cream, and Amy were the first to find her, and they questioned her arrival. Cosmo told them that she was looking for the "legendary hero that can use the Chaos Emeralds, Sonic the Hedgehog." However, she refused to tell them any more. Later, while in Tails' jet, the X-Tornado, she spotted Sonic and hopped out to greet him. Cosmo pleaded for Sonic to help her.
Sonic agreed to help her and he, Cosmo, Tails, Knuckles, Amy, Cream, Cheese and Chris Thorndyke, who recently arrived on Sonic's world, began traveling through the universe in Tails' new space ship, the Blue Typhoon in search of the Chaos Emeralds which Sonic scattered through the universe during his first battle with Dark Oak.
Later in the series, Shadow boarded their ship and attempted to kill Cosmo, as he believed her to be a spy for the Metarex, but Cosmo's friends managed to keep her safe. Tails especially endeavored to keep her safe, to the point of almost being knocked unconscious. Before Shadow could do Cosmo any harm, a Metarex broke in and interrupted the fighting. Dark Oak called Cosmo "White Seed" and told her that she did a good job. Cosmo then remembered that before she escaped from her space colony, the Metarex were planning to use her as a spy.
Chris and Tails managed to locate an invisible signal and found that it was attached to her brain; if they had gotten rid of it, Cosmo's hearing and sight may have been lost in the process. In the English version Knuckles said that it would have been best to do that, but Tails refused to harm Cosmo. In the Japanese version, however, Knuckles yelled at Tails that there had to be a way to remove it without hurting Cosmo, to which Tails yelled back that he had no idea how to do both, assuming it was possible.
Later, in the English version, Cosmo learned that she could hear the Metarex in her head and fell into deeper despair. While she was disheartened, Chris told Sonic that he now understood why Cosmo was able to understand the Metarex language.
In episode 74, Cosmo tried to run away but was stopped by Tails. Their friends then showed up and said they were determined to defeat Dark Oak instead of risking Cosmo's hearing and sight. In episode 77, Cosmo sacrificed herself by fusing herself with the Dark Oak planet to make it weaker so that Tails, Super Sonic and Super Shadow could destroy it and save the universe. At first, Tails refused to, but Cosmo convinced him that it was the only way to save the galaxy. Tails trusted Cosmo and ended up shooting her (in the Japanese dub, he yelled that he loved her while doing so) with the Sonic Driver (Sonic Power Cannon in the English dub), causing her to be killed in the process (in the ending credits, she says to Tails that she loves him, while in the dub she says she'll never forget him). According to the Japanese version, in an attempt to rescue Cosmo from death, Super Sonic and Super Shadow used Chaos Regeneration, but all they managed to produce was a small seed although in the English version, Chaos Regeneration is not used and Cosmo implies that the seed is her child. In episode 78, when Sonic arrived back on board the Blue Typhoon, he hands the seed to Tails, who collapsed with grief as he realized that Sonic was unable to save Cosmo. However, the seed sprouted in the end.
Cosmo is a kind and gentle character, though she often feels guilt because she was unable to save anyone from her home planet ever since the destruction of the Metarex. She also seems to suffer from a low self-esteem issue, often calling herself "unworthy" or blaming herself for the hardships that she and her friends had to endure. Cosmo also dislikes violence, but acknowledges that fighting is sometimes a necessity, although she herself is unable to fight. Cosmo can be somewhat clumsy, but she is also quite resourceful; she also has a penchant of getting into trouble.
Cosmo shares similar qualities with both Maria Robotnik and Tikal the Echidna. Both Maria and Tikal sacrificed themselves to save their friends/beliefs and are pacifists, but know how to fight when necessary.
Cosmo does have a few abilities, being a plant herself, such as having the ability to empathize with plants, trees, or the planet and determine their state of life. Cosmo can jump high and gently float down with the aid of her skirt. She also appears to be able to unlock the power of a Planet Egg, as demonstrated in episode 58, where she caused the Planet Egg to activate and grow vines that entangled Doctor Eggman's machines, but also her friends and herself due to her lack of control over the vines. In episode 59, Cosmo has demonstrated some sort of agility as shown when the Chaotix were about to leave and kidnap Cream with a sack, she quickly rolled next to Cream letting herself get captured along with Cream and later on when Vector and Cosmo get into a fight, she is able to avoid his attacks with speed, leaving Vector only to hurt himself before Sonic came to their aid. She's able to read and decipher the Metarex language (most likely given to her when the Metarex implanted the signal while she attempted to escape) When Cosmo is in her super transformation, she is able to turn into a tree and weaken other planets.
Cosmo's weaknesses include being physically weak and acting a little clumsy. She has trouble carrying heavy objects but can use certain objects as defense. She also shows heavy endurance as she is able to recover from powerful attacks quickly as seen in episode 58 where she quickly arrived back to attack a Metarex with fruits after being sent flying into the air by a heavy branch shaped like a club. Cosmo also tends to get into trouble easily. She also might have air sickness as when Tails was flying high above Angel Island, she was shown to be very dizzy and when Amy was trying to catch up to Sonic, she sped up her ship but Cosmo wasn't feeling well so Amy held back.
On Sora's TeamEdit
Her spirit sometimes appears and helps the team with guidence, and what is the right way and the wrong way. Even scare enemies who are scared of ghosts.