Little is known of Zirconia's motivations in the manga.
She doesn't like the light of day and only feels comfortable
in darkness. She is the one who Neherenia talks to from behind the mirror,
and who gives their orders to the Amazoness Quartet.
She first charges the Quartet with making sure the circus is a success, and
then with taking over everything for the Dead Moon Circus, little by little.
She instructs them not to attack the powerful Senshi out front, and to proceed
cautiously. She warns them failure will not be tolerated. As an example,
she gives them an earful when SailorMoon and ChibiMoon break free of
PallaPalla's Opposite Ball spell. (The spell where
their ages were switched, and Usagi becomes Chibi-usa's age and vise-versa.)
She tells them that to conquer Earth, they must first destroy the enemy, and she
orders the Quartet to fill the Senshi with nightmares.
Her role becomes more important during the battle in which the Outers finally appear. Just
as Saturn succeeded in freeing the Amazoness Quartet from Zirconia's influence,
the four girls were trapped in their balls by Zirconia. She then proceeded to trap
Saturn and ChibiMoon in two shards of glass from a broken mirror. She sends the balls and
the glass to the other side of the mirror, where Neherenia laughs as the items
fall at her feet.
Zirconia's other moment of glory, so to speak, arrives when Helios has used the last
of his strength. Zirconia traps the senshi and Sailor Moon in nightmares, making it
look to the senshi like she has killed Sailor Moon and Tuxedo Kamen, while making it
look to Sailor Moon like all the senshi are dead, even making her believe that she is
dying herself. Unfortunately
for Zirconia, Tuxedo Kamen manages to slap the senshi awake. They do a group attack on
Zirconia, hurting her and forcing her to run away. She goes inside the mirror, followed
by SailorMoon, who only finds Neherenia on the other side.
After Neherenia seems to have been defeated, Zirconia makes a flash appearance to
cackle a bit and say that Neherenia will not die yet. It's Zirconia's last appearance,
unless you believe that what Neherenia turns into when she finally is defeated
is indeed Zirconia.