Comments from the second tankoubon:

She is the Pillar that supports Cephiro with the power of her Prayer. Cephiro reflects exactly what is going on within the Princess's Heart. Right now, she cannot pray for the peace and stability of Cephiro, so there are monsters creating confusion and war all thoughout the land. Although younger than Clef, she is pretty old. Nevertheless, the Princess looks like a young girl.

<ChunHyang> How does she look so damn young?  She
            looks younger than all three Magic Knights!
<Satsuki> WILLPOWER.
<ChunHyang> So she WANTS to look
            that young?
<Asuka> I would rather be a voluptuous, tempting
        twentysomething like Alcione!
<Rei> She can't fall in love, that would be a bad
      idea.
<Asuka> You are no fun. 
<Alice> So why is she imprisoned?
<Yang> Supposedly Zagato took her and imprisoned her.
<Shinji> Actually...
* Asuka muffles Shinji.
<Asuka> BAKA!  Don't spoil it!
<Shinji> Well, if you want to be spoiled.....
<Rei> Wait until we talk about Zagato, then we can
      talk about the truth behind them.
<Shinji> Okay.