|Lives:||Galaxy Cauldron, Sagittarius Zero Star|
|Aliases:||Chaos Galaxia (anime only), Sailor Chaos (manga and musicals only), Kyaosu (musicals only)|
|First Anime Appearance:||Usagi's Love! The Moonlight Lights up the Galaxy|
|First Manga Appearance:||Act 51 Stars 9|
|First PGSM Appearance:||N/A|
In the anime, Sailor Galaxia had sealed Chaos within her body, and was eventually taken over by it. Although Chaos as a character was never referred to by name in the anime, when Galaxia's bracelets shattered in episode 200 and she changed into the form known as Chaos Galaxia, it showed that Chaos had completely taken over her body and she had, essentially, become Chaos.
After Sailor Moon freed Galaxia from Chaos' influence, it left her body and returned to its rightful home, within the hearts of people everywhere.
In the manga, Chaos was the being responsible for seducing Galaxia with promises of power. It was eventually revealed that all the other main villains that Sailor Moon had battled up to that point - Queen Metalia, Death Phantom, Pharaoh 90, and Queen Nehellenia - were simply incarnations of Chaos.
Chaos referred to itself as one who never became a star. It existed as the ultimate expression of nothingness, but still remembered the time when everything in the universe was one within the Galaxy Cauldron, and it missed that desperately. Most of all it missed the ultimate light.
Chaos made it its goal to once again become one with everything else in the universe. To this end, it wrought war, death, destruction, and hate, destroying everything so that they could return to it. It created other "dark stars" like itself - Metalia, Death Phantom, Pharaoh 90, and Nehellenia - and sent them against Sailor Moon, forcing the one who fought in the name of love, peace, and justice to destroy beings that were essentially her own brothers and sisters.
When Sailor Moon reached the Galaxy Cauldron, she discovered that Chaos had merged with it. This meant that destroying Chaos required the destruction of the Cauldron as well, which would prevent new stars from being born. Sailor Chibi Chibi Moon told her to destroy the Cauldron in order to destroy Chaos and thus ensure peace in the galaxy, but Sailor Galaxia disagreed, saying that if this Cauldron was destroyed another would simply come into being somewhere else. Sailor Moon chose to instead preserve the Cauldron, and Chaos as well. She plunged into the Cauldron, taking Chaos with her, which dissipated Chaos' essence enough that the galaxy was given a temporary reprieve. Chaos would return eventually, but by saving the Cauldron, Sailor Moon had ensured that the stars would not die out, and there would be a future for the galaxy.
According to Sailor Cosmos, eventually Chaos returned in the form of Sailor Chaos and began an incredibly destructive war that devastated the galaxy. This was the reason that she went back to the past to ensure Chaos' destruction, but Sailor Moon's actions gave her hope for the future once more.
In the musical Eien Densetsu and its revision, Chaos (played by Ryuuji Kasahara) was originally portrayed as merely one of Sailor Galaxia's bumbling sidekicks, at which point he was referred to as Kyaosu (キャオス). Kyaosu did not seem to be entirely loyal to Galaxia and was willing to help Sailor Moon out at one point.
After Sailor Moon freed Galaxia using the power of the Light of Hope, Chaos revealed its true nature and tried to take the Silver Crystal. The freed Sailor Galaxia returned and saved Sailor Moon, and after she was joined by the other Sailor Senshi, they defeated it.
In Greek mythology, Chaos was a primordial god, a dark shapeless void, and the original state of existence from which the first entities appeared. The mythological Chaos was neither good nor evil, and was rather just the ultimate expression of nothingness and the antithesis of Cosmos.