Guardian Cosmos makes her first and only appearance in Act 52, Volume 18 of the manga.
She never appears in the anime. Sailor Moon dives into the Galaxy Cauldron to rescue her
loved ones, and as she enters, her body melts away. The Sailor Quartetto look on, horrified,
but Sailor Cosmos explains to them that all of the Sailor Crystals within the Cauldron will
be safely reborn. Inside the Cauldron, Usagi is reunited with her friends. 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..."
Guardian Cosmos is often incorrectly called the "Cauldron Guardian." While she IS the guardian of the Cauldron, she gives her name as Guardian Cosmos, and this is what she should be called. Contrary to popular belief, the Cauldron Guardian is NOT Chaos.