Chaos was the energy that had taken over Galaxia's body. It wanted to take all the Sailor Senshi's star seeds. At the end of the series Chaos completely took over Galaxia's body, which turned her black, gave her red eyes, and gave her black bat-like wings.
Chaos was destroyed when Sailor Moon convinced Galaxia to take over her body again. Sailor Moon said that Chaos had returned to its place where it had been before, in the hearts of everyone.
Galaxia is the ruler of the evil Sailor Senshi, and always says that the Galaxy belongs to her. In the beginning of Sailor Stars she released Nephrenia and gave her power, so that the battle would revive Sailor Saturn.
In the beginning of the season she captured Tuxedo Mask and took his star seed, because he was the senshi of the earth. The Senshi didn't realize he was gone until the end of the season, since he was going to America for a big oppurtunity.
Sailor Galaxia had taken all the star seeds, including Sailor Moon's, when Chibi Chibi Moon used her power and saved Sailor Moon. Galaxia realized that that was the "light of hope"!
In an earlier battle Galaxia had defeated Chaos and sealed it inside of her, but she knew Chaos would eventually take over her body. She sent away her star seed, the "light of hope", to a place safe. That was the milky way.
Sailor Iron Mouse worked under Galaxia. Her job was to find the star seed that Galaxia was looking for. She liked to wear pin-striped outfits a lot, and traveled in a police box? (Tardis... many believe this is a pun of Dr. Who)
She tried to take Sailor Star Fighter's star seed, but she had failed so many times that Galaxia stopped her and killed her by taking her bracelets.
Aluminum Siren took over after Iron Mouse was killed. She was one of the Anima-mates.
Siren and Lead Crow, despite being rivals, were good friends. Lead Crow was often concerned for Siren, and would push her to find people with star seeds so Galaxia wouldn't be angry at her. However, Siren wasn't that enthusiastic about getting star seeds. She would also eat whenever she became disappointed or depressed.
One time, she attacked Eternal Sailor Moon with really long finger nails, and tried to take out her star seed, but Sailor Chibi Chibi saved Eternal Sailor Moon.
She was killed in episode 188 when Galaxia took away her bracelets (containing her sailor crystal). However, Siren had written down in her diary that she knew Eternal Sailor Moon's identity, and later Lead Crow and Tin Nyanko would read that diary...
Sailor Lead Crow is number one in the Anima-mates, and was a rival with Aluminum Siren. When Siren was sent to earth, Lead Crow followed to keep an eye on her.
She was very sad when Sailor Aluminum Siren was killed. She was also very angry with Sailor Tin Nyanko, who had encouraged Sailor Galaxia to destroy Sailor Aluminum Siren. She also hated Sailor Tin Nyanko, and constantly argued with her.
One day she found Sailor Aluminum Siren's diary, and found out the true identity of Sailor Moon. She attacked Usagi and almost took away her star seed, but Tin Nyanko attacked her. The capsule broke (containing the star seed) and she was killed by the black hole that came out of it.
Sailor Tin Nyanko took over after Sailor Aluminum Siren was killed. She and Sailor Lead Crow constantly got into arguments, as Tin Nyanko was encouraging Sailor Galaxia to destroy Sailor Aluminum Siren. She was probably most responsible for Lead Crow's death, since her attack caused the black whole to open, which killed Lead Crow.
She kept trying to get the star seed from Usagi, and almost did twice, but Galaxia appeared and took away her bracelets in ep. 195.
Galaxia offered Uranus and Neptune that if they took her bracelets when she took their star seeds, they would live forever. Uranus and Neptune agreed because, even though they would be "in hell", they would be together.
They took Saturn's and Pluto's star seeds, and fought the Starlights, but when Galaxia powered them up to take Sailor Moon's star seed, they turned on her and tried to take hers. It didn't work, so Galaxia took away their bracelets and they faded.