She's an ally of Sonic the Hedgehog and his friends.
It's widely accepted that her primary purpose in the series is that of a love interest (but not girlfriend) for Tails, the two characters being of the same age and displaying an overt fondness for one another. Cosmo the Seedrian is the love interest (But not girlfriend) of Miles "Tails" Prower.
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 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 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.
Then, Cosmo pleaded for Sonic to help her. Sonic agreed to help her and he, Cosmo, Tails, Knuckles, Amy, Cream, Cheese and Chris, 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. Then, Cosmo 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, Knuckles yelled at Tails that there had to be a way to remove it without hurting Cosmo which Tails yelled back that he had no idea how to do both, assuming that 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 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 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.
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, meaning that resurrection of Cosmo could be possible. The episode and the series ends as the pod with Chris in it flies out into space and then twinkles out.The result is she is dead but later return at the second movie.
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 Maria Robotnik and Tikal the Echinda.
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 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 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's 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 was 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 speed up her ship, but Cosmo wasn't feeling well so Amy held back.
The only confirmed members of Cosmo's family are her mother Hertia (Earthia in the English dub) who is the leader of the Green Planet council and her older sister, Galaxina who worries about Cosmo and her mother.
Earthia/Hertia: Her mother, who is the leader of the Green Planet council. She decides to leave the planet when it becomes too dangerous, but is later killed after the Metarex board her refugee ship.
Galaxina: Her older sister who's worried a great deal about Cosmo and their mother, Hertia.
She is portrayed as very smart, sophisticated, and down-to-earth, the opposite of the dreamy Cosmo.
Seedling: Possibly Cosmo's child or Cosmo turned into a seed, a seed that was given to Tails by Sonic in episode 78, after Cosmo's death.
- Because they are plant-like creatures, it is possible that the members of Cosmo's species can be reborn.
Cosmo became good friends with Sonic and friends, but most particularly with her boyfriend Tails.
The two of them ended up alone on a mission together at one point when Tails got hurt, Cosmo stiched his bruise and Tails blushes in return.
Later on, when the Blue Typhoon meets the Chaotix for the second time.
In return for Tails repairing the Chaotix's space ship, the Chaotix could already tell that Tails is starting to have an affection for Cosmo, so the trio tried plans to join the two together in love.
Although those plans failed, Tails and Cosmo's relationship seemed to be getting stronger even to the point of Tails saying "Cosmo, you're so beautiful", but didn't had the courage to completely say it and Cosmo misheard it, thinking about something else.
Their love for each other is confirmed in the episode 'The Planet of Misfortune' when the Blue Typhoon lands at an alien planet, because Amy discovered that the planet had magic especially fortune telling, love charms and a special, enchanted lake that when a couple throws themselves into that lake, their love for each other will be very strong and the couple is said to be united forever.
These were the things to have her and Sonic together in love while Cream inquires about Cosmo's future and the fortune teller states that there is romance in her future.
He also says it's strange since 'Plants and animals do not usually get along.' which forshadows future events throughout the series between Cosmo and Tails.
Although Amy has set-up a trap to lure Sonic into the lake, Tails and Cosmo accidently trigger it themselves, leaving the duo in love for each other while watching the moon, much to Amy's disgrace.
Tails later protected Cosmo when Shadow targeted her for assassination. Tails continued trying to stop Shadow even when he was very badly injured by him.
In the Japanese version of episode 77, everyone is too shocked to realise that to defeat and vanquish Dark Oak and the Metarex forever.
Cosmo plants herself on Dark Oak (in her metamorphosis form), telling the heroes to fire the Sonic Driver/Power Cannon at Dark Oak, but this involved killing her as well much to everyone's dismay especially Tails.
When it's up to him to fire the Cannon, Tails was extremely hesistant to do so, but with the help of Eggman's reassurance, Tails screamed "I love you, Cosmo!" just before he fired the Sonic Driver towards her and Dark and it's strongly implied throughout the episode that Tails simply didn't have the courage even at the end, to admit that he loved her (this was only in the Japanese version).
- In the English dub, Shadow believed that Cosmo was a traitor, despite being the very person who called the Sonic & co. to this mission. However, this is most likely the only reason 4Kids could find to cover up Shadow's violent actions in the Japanese version.
- In Sonic Chronicles, Tails' Workshop contains a potted plant very similar to the one shown at the end of Sonic X.
- As shown in Episode 59 of Sonic X, when Charmy appeared dressed up in what looked like trash, everyone attacked him mistaking him for a Metarex as well as Cosmo.
- Cosmo's family's names are derived from astronomical terms (Cosmos, Galaxy, etc).
The Flower Buds on Cosmo's head are roses.
- Despite her official name being "Cosmo the Seedrian", Cosmo is sometimes called "Cosmo the Plant" by fans. This is probably due to the Seedrian race being anthropomorphic plants.
- While Cosmo is liked by most Sonic the Hedgehog fans, there are some people who dislike Cosmo. This is usually based on fan-prefered pairings; some people tend to support "Taiream" (Tails and Cream as a couple), while others support "Tailsmo" (Tails and Cosmo as a couple).
- While there's an almost equal amount of both supporters, it's more likely for the "Tailsmo" pairing to work, as Tails was confirmed to have a crush on Cosmo and was never shown to have an official crush on Cream.
- Cosmo the seedrian has a boyfriend called Miles "Tails" Prower.
According to answers.com, Cosmo turns into a seed in the Japanese version due to Chaos Regeneration... http://www.answers.com/Q/What_happened_after_Cosmo_died_on_Sonic_X