Don't remember it all clearly, but she was a former Sailor Senshi. She decided to get a hold of all the other sailor's crystals and become the most powerful.
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Spoiler: good 'ol wiki
Sailor Galaxia (セーラーギャラクシア, Sērā Gyarakushia?) emerges as Sailor Moon's strongest opponent in the manga Stars and its anime counterpart, Sailor Stars. As such, she serves as the main antagonist in the above-mentioned story arc. As the self-proclaimed "Golden Queen of Shadow Galactica" she directly commands the Sailor Animamates. She is also described as the Soldier of Solitude.[1] Her goal is to discover all the Sailor Crystals in the universe so that she can rule the universe unopposed. Galaxia's own crystal, the Saffer Crystal, is described as one of the most powerful Sailor Crystals in the galaxy. Its name is Old English for "sapphire,"[2] and the kanji applied to it in the text of the manga are 青金石 (seikinseki), meaning "lapis lazuli."
In the manga, Sailor Galaxia originated alone on a small planet she deemed "trash". She wandered alone until the day her powers awakened. But she had no purpose for herself, and felt that being just a Sailor Senshi did not satisfy her; 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, Chaos in the form of Wiseman, 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 Soldiers. In her first conquest on Earth, she stole 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.
In the anime, Sailor Galaxia once ranked as the legendarily strongest Sailor Senshi in the universe. The Sailor Wars pitted all of the Sailor Senshi against those who infringed against 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 sealed up. Knowing her power put the entire universe at risk, Galaxia expelled her star seed (in the form of ChibiChibi) into the rural galaxy of the Milky Way, where the newly born stars of love and justice resided.
With that her acts of free will came to an end. Now completely controlled by Chaos, Sailor Galaxia sought to steal all the star seeds in the universe in order to control it for herself in the name of "Star Galaxia". The planets she wrecked included the planet Kinmoku; the resident Sailor Soldiers and Princess Kakyuu managed to escape destruction and flee to Earth, the last place that Sailor Galaxia had not invaded.
In order to enter Earth, Sailor Galaxia did two things first. She broke the seal of Queen Nehellenia in order to force the awakening of the soldier of silence, Sailor Saturn. Galaxia wanted to have Sailor Saturn ready for Galaxia to take all the star seeds of the Solar System. After Nehellenia was defeated by Sailor Moon and she was given a second chance at life, Galaxia began her plan. As Mamoru Chiba was flying to America to begin his studies, Galaxia attacked the airplane he was travelling in and stole his golden star seed. Once this was done, she sent her Sailor Animamates to find true star seeds belonging to Sailor Soldiers - in other words, Sailor Crystals. After numerous failures, Galaxia would kill most of her Animamates, and eventually confronted the Sailor Soldiers herself.
Galaxia first battled Princess Kakyuu, who had recently come out of hiding. While Kakyuu succeeded in holding a barrier against Galaxia for a time, her enemies eventually overpowered and killed her. The Starlights, Guardian Senshi, and Outer Senshi each initiated their own plans of attack. The Starlights confronted Galaxia first, but before the loss of their star seeds the Guardian Senshi teleported in to rescue them. During Galaxia's attempt to steal the star seeds of the Starlights, the Guardian Senshi intervened directly, protecting them from attack and dying so that their star seeds were stolen instead.
With the battered Starlights protecting Sailor Moon and Chibi Chibi, the Outer Senshi confronted Galaxia. Now Galaxia revealed her hand in releasing Nehellenia and forcing Saturn to mature. Galaxia offered the Outer Senshi a chance to serve her in exchange for their starseeds — an offer which Uranus and Neptune "accepted", killing Pluto and Saturn and then proceeding to hunt down the Starlights. With their own powers enhanced by Galaxia's power, Uranus and Neptune succeeded in overpowering the Starlights, but turned on Galaxia at the last minute to try to take her starseed. Their attack failed since Galaxia had no starseed to take and the last two Outer Sailor Soldiers were killed.
The Starlights engaged in a final battle with Galaxia, teaming up with their attacks and managing to do some damage to her before going down in defeat. Unlike most of the other characters, though, they did not die. Galaxia crushed Sailor Star Fighter's hand and Sailor Star Yell with her heel before the Sailor Soldiers could try another assault, gloating that apart from the five in front of her - the three Starlights, Sailor Moon, and ChibiChibi - every other being in the galaxy had had their starseed removed. Sailor Moon tried to restore her to normal but her attack was stopped when Galaxia broke her scepter. In an attempt to defend the fallen Starlights, Eternal Sailor Moon's star seed was taken by Sailor Galaxia. Here, ChibiChibi's powers awoke revealing her to be the Light of Hope. She stops Galaxia's attack and restores Sailor Moon's star seed. Sailor Moon learns here that Chibi Chibi is the re-incarnation of Galaxia's star seed. Sailor Moon with her star seed becomes Princess Serenity and the Light of Hope transforms into the Sword of Sealing, which she asks Serenity to use as a weapon to defeat Galaxia. When Serenity is unwilling to do so, Galaxia shatters the sword and ChibiChibi disappears.
Sailor Moon continues to refuse to fight Sailor Galaxia, believing that she still has good inside her. She instead 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. The battle finally ends: Sailor Galaxia's soul having been restored and the star seeds she had stolen released. Galaxia then leaves with the task of returning the star seeds home.
In the anime, Mitsuko Horie voiced Galaxia.[3] In the stage musicals, four different actresses have portrayed her: Saori Sara, Tamami Matsumoto, Takemi, and Yumie Sakaguchi. Matsumoto stated that she greatly enjoyed the role: "ordering people around and being above others is such a great feeling".
Spoiler: Sailor Moon 
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