1. Magic Knight
The mission of the first three volumes is solid from the beginning, even if they do hide the true meaning of saving the princess. The Magic Knight has to be from another universe, to be able to make the pillar's wish come true.
Hikaru from most of the first three volumes believes that her job is saving Emeraude, because she has been kidnapped by Zagato, and that he is the evil behind everything.
But in the end, when the last battle begins, Hikaru, along with Umi and Fuu, find out what their true mission is, and while it is freeing Princess Emeraude, that means killing her, because nobody else can do it.
"If anything would happen to the Pillar that support is, Cephiro
itself would be destroyed. That's why the Pillar alone has a special power of
summoning. A power to bring Knights from a world other then Cephiro... in order
to kill oneself..."
-- Princess Emeraude; vol 3, p.180
Pillar's will is what keeps Cephiro alive, or better in existence. The Pillar must care only for Cephiro and nothing else. It's the Pillar's prayers that keep the harmony of the world, and the person who becomes a Pillar must have a really strong will to do so. So it's not strange that Hikaru became one.
With a small difference from her predecessor, Emeraude, Hikaru, when she becomes Pillar decides to abolish the pillar based system, also because that means calling future tragedies, and with her power she gives the place in hand of all the people who love their "country".
"The princess could have eliminated the Pillar System. Instead
she chose to die and continue this system... where one decided all and there
is an unchanging tomorrow. But the new Pillar of Cephiro... has wished not for
her death, but the death of the System"
-- Mokona; vol 6, p185