Galaxia was once a normal woman living in a galaxy far from Earth. During the Sailor Wars, she was possessed by Chaos and turned evil. She went around everywhere, collecting Star Seeds, looking specifically for some very powerful ones (which turned out to be the Senshi's) to take over the galaxy with. She didn't do the job herself; she sent out women known as Animamates, who she'd taken the Star Seeds from, to do the job for her. She wore gold bracelets which she gave duplicates of to the Animamates to control them with. Her Animamates were all killed by the Senshi, and Eternalsailormoon healed her, casting out Chaos.
In both the anime and the manga, Galaxia is a vicious, cruel ruler. She treats her Animamates as if they were no more than her servants. She will stop at nothing to take over the galaxy. The Senshi can't harm her, it appears; they attack and she blocks it as if they were just minor annoyances.
In the anime, before Galaxia was possessed, she released her Star Seed, which became Chibi-Chibi. In the final battle with Sailormoon, Moon heals her, releasing Chaos and returning her to her original form.
In the manga, Galaxia has destroyed many planets, and has more Animamates than in the anime. She toys with Moon's feelings, and is very manipulative. She restores the dead Senshi as her servants, and they attack Moon while under her control. She kisses Tuxedo Kamen when under her control, and procedes to push him off a cliff while Moon watches. Galaxia's bracelets shatter sometime after Chaos leaves her body, and she dies.
Galaxia has long, long hair, shading from golden at the top to orange then red. You don't see it often since she's wearing that headdress. Her eyes appear to be a red color. She's fairly tall. You always see her in her uniform, but she briefly sheds it in the manga.
Galaxia doesn't seem to have any particular powers, but she does have a lot of it. She always wears golden armor; a fancy headdress, gold plated dress, and tall boots, plus her bracelets and a brooch on her chest.
*falls to knees and grovels* Galaxia is one of the best villains ever! She is so evil! Not to mention the great costume, voice (gotta love the "Golden Queen Galaxia" song!), and the hair! Wow! They almost... dulled her somewhat in the anime (she didn't even die! Not that that's a good thing...) Part of the time, I'm screaming "Yay, Galaxia!" and the rest "If you weren't so cool, I'd hate you for doing that!" Basically, a really great character.
Senshi Name: Sailorgalaxia