Well thank you for enjoying the storyline so far.
Now to answer your questions one by one.
1. I suppose Siele is going to use her as a sort of weapon, right?
Siele has very special plans for Liltaia, she won't use as a weapon outright, but the power that is within Liltaia has the ability to destroy living creatures with only a look. So you can make up a storyline in your head.
2. How is supposed Litlaia to leave Loielis without being stopped by guards? I feel you'll come out with some good subterfuge:)
Her leaving Loielis will awaken powers in her, since the only way to escape the barriers is by using the Green Flame, which was awoken when the guards tried to kill Aaila.
3. Siele is a completely evil character or has some different aspects in her personality?
She is a bipolar character, based of some people I've met at previous times in my life. Yes, she is evii and somewhat monstrous, but she was always sweet, kind, sincere, and endearing to Liltaia as if she were her own child. But she kept Liltaia hidden within the confinements of the castle. This in turn alone cause damage to Liltaia and Siele's relationship.
4. I understand Litlaia is so willing to helpe someone, coming out from her "golden cage", but isn't stopping at a village to free it from monsters while Hell Guards are looking for her, a countersense?
At that time, Aaila and Liltaia still thought that the Hell Guards hadn't been sent yet. So, they thought they were safe or at least hidden. That part was made to be a decision of selflessness, but one also of naive thinking.
5. Is the book you mentioned a sort of prophecy book, or is more likely a Bible, or a sort of study conducted by some alchemist on Litlaia?
It is a mixture of a prophecy and a study, but the study wasn't conducted on Liltaia herself. You will see when the game is truly complete.
6. Have you already an idea of the characters' appearance?
Yes, I'm just not good at making sprites so I might have to get someone to make the sprite for me. Then I'll upload screenshots.
If you have any other questions please ask, I'll answer them to the best of my abilities without really exposing the secrets.