Guardian Cosmos

Guardian Cosmos is the guardian of the Cosmos Crystal and the Galaxy Cauldron. In the fanseries, Cosmos is the one who summons both Hikari and Kiyomi to the Sailor Moon universe to "fix all the wrongs in this world".

Appearance
Guardian Cosmos is a small female spirit with long hair who wears a long transparent dress decorated with beads. She wears a tiara accessory on her head and carries a staff.

Pretty Soldier Sailor Moon Stars (Original Timeline)
Sailor Moon dives into the Galaxy Cauldron to rescue her loved ones, sacrificing and expecting to melt away like all others who have fallen into the Cauldron. Her body remains whole and she is immediately greeted by a small figure of a woman. The woman introduces herself as Guardian Cosmos, the guardian and holy spirit of the Cosmos seed. She wonders aloud if Usagi and the others wish to begin a new history of the stars, and leave the Cauldron in their star forms, but Usagi tells her that no matter how hard it will be, she wishes to return to the way she and her friends knew each other, so that they may continue to live on together as friends.

They were encased in light and begin to float away. As they float away, Usagi calls out to Guardian Cosmos, saying, "Is Chaos...?" but is cut off, and disappears. After she disappears, Guardian Cosmos says, "The core of Chaos, the Chaos seed, and Guardian Chaos, are melted into the Cauldron. Now they have become too small to be found. They might be born again, because this is the place where stars, and possibilities, are born...".

Pretty Soldier Sailor Stellar
In the fanseries, Guardian Cosmos has the power to see into alternate universes, and has been watching Hikari for a very long time. Recognising her potential for being a Sailor Senshi, she meets with Hikari and teleports her to the Sailor Moon universe.

Trivia

 * Usagi's encounter with Guardian Cosmos bears some notable similarities with the most forgotten Disney film The Black Cauldron.