In the anime, Sailor Galaxia was once the legendary strongest Sailor Senshi in the universe. The Sailor Wars pitted all of the Sailor Senshi against those who infringed on love and justice. Eventually, the Wars ended when Sailor Galaxia sealed the source of all malice, Chaos, within her body to save the cosmos. However, Chaos began to increase its power while it was sealed. Knowing her power put the entire universe at risk, Galaxia expelled her star seed into the rural galaxy of the Milky Way, where the newly born stars of love and justice resided. It was her last act of free will. Now completely controlled by Chaos, Sailor Galaxia sought to steal all the star seeds in the universe to control it for herself. In the end Sailor Moon gives Galaxia the strength to expel Chaos from her body, an act which returns darkness to the minds of all life, where it belongs together with the Light of Hope. Galaxia then leaves with the task of returning all the star seeds home.
In the manga, Sailor Galaxia was born alone on a small planet she deemed "trash". She wandered through alone until the day her powers awakened. But she had no purpose for herself, and felt she needed something more in life. Even being just a Sailor Senshi was not enough; she craved something and she thought that was power. She left her planet in search of another place which she could call home. During her travels she came upon a bar, talking to someone there she asked about the place "where stars are born". Not finding an answer, she determined that everyone there was "trash". But someone caught her attention when it spoke of the holy land she was searching for. That someone was Chaos in the form of Wiseman. Chaos showed Galaxia what she was looking for—the Galaxy Cauldron. She discovered that in order for her gain control of the Galaxy Cauldron she had to gather the Sailor Crystals of all the Sailor Soldiers of the galaxy. Those powers combined together with that of Sailor Moon would give her the power to kill Chaos and rule the galaxy. She then created Shadow Galactica and traveled to planet after planet turning them into places of death and stealing the star seeds of their guardian Sailor Soldier. Stealing someone's star seed allowed her to revive their dead body, and use it like a puppet, such as the solar system's Sailor Soldiers. Her first conquest on Earth was to steal Mamoru's golden star seed right in front of Usagi's eyes; and after that, she and her minions stole the life force of each of the Sailor Soldiers on Earth until only Sailor Moon, Chibichibi and the Sailor Quartet were left. At the Galaxy Cauldron, Galaxia proceeded to battle them, until Chaos tried to kill Galaxia by blasting her off a cliff. Sailor Moon rescued her from falling over the cliff into the cauldron, and Galaxia finally realized that what she had always really wanted was love, not power. By accepting Sailor Moon's offer of friendship, the bracelets which supplied Galaxia with life and power shattered, and she died with a smile on her face.