Cosmo is a character introduced in the anime Sonic X, specifically appearing as a main cast member during the series' third season. She is a young, humanoid alien belonging to a plant-like species known as the Seedrians. Her home planet was destroyed by a villainous faction called the Metarex, an event that left her as one of the sole survivors of her kind. Following this tragedy, Cosmo escaped through space in a vessel that eventually crash-landed on Sonic the Hedgehog's world, where she was discovered by Tails, Amy Rose, and Cream the Rabbit.

In terms of personality, Cosmo is consistently portrayed as gentle, kind, and reserved, though she carries a significant emotional burden. She struggles with low self-esteem and often blames herself for the destruction of her people and for the troubles her new friends endure. She frequently criticizes her own abilities and feels unworthy of help. Despite her quiet nature and deep aversion to violence, Cosmo possesses a strong sense of duty and shows remarkable resourcefulness. She cares deeply for her friends and is driven by a desire to atone for her past and stop the Metarex's conquest.

Cosmo’s motivations are rooted in survival and redemption. She seeks out Sonic because she believes he is the legendary hero capable of using the Chaos Emeralds to defeat the Metarex. She joins the crew aboard the Blue Typhoon, the spaceship built by Tails, and becomes instrumental in the fight against the Metarex, using her unique knowledge of their species and tactics to aid the team. Her role in the story is that of a guide and a key to understanding the enemy, though she is not a frontline fighter.

Her most significant relationship is with Miles Tails Prower. Over the course of their journey, a deep romantic bond forms between them. Tails is protective of her, and she finds comfort and strength in his support. Other key relationships include a close friendship with Amy and Cream, who offer her emotional support, and a respectful alliance with Sonic, whom she views as a hero. Her relationship with Shadow the Hedgehog is tense, as he initially sees her as a threat and attempts to eliminate her upon discovering she is an unwilling spy for the Metarex.

Cosmo possesses several notable abilities tied to her plant-like physiology. She is very light and can use the petal-like extensions of her dress to float gently down from great heights. She has an empathetic connection to plant life and planetary ecosystems, allowing her to sense their health and vitality. Cosmo can also unlock the power of Planet Eggs, causing rapid vegetation growth. She is able to read and decipher the Metarex language. A red gemstone amulet, left to her by her mother, serves as a source of power. In a heightened state, she can transform into a large tree-like form, a powerful but self-sacrificial ability that can immobilize massive targets and weaken planets.

Throughout the series, Cosmo undergoes significant emotional development. She begins as a timid, self-loathing survivor burdened by guilt but gradually learns to believe in herself and her ability to contribute. The revelation that the Metarex planted a tracking device inside her brain, making her an unwitting spy, is a major crisis point that deepens her despair. However, with the encouragement of her friends, particularly Tails, she resolves to face her destiny. Her development culminates in a final, heroic act of self-sacrifice. In the climactic battle, Cosmo transforms into a massive tree to restrain the Metarex leader, Dark Oak, allowing Super Sonic and Super Shadow to destroy him. She persuades a grief-stricken Tails to fire the weapon that will end the battle, knowing it will also end her life. After the explosion, a small, single seed is all that remains of her, which Sonic gives to Tails. The seed is later seen sprouting in a pot in Tails's workshop, leaving her ultimate fate ambiguous.