Queen Sia was the queen of the Realm, and the main antagonist of the series. She was the second wife of the (late) king, making her Finn/Giles' stepmother, and Prince Caspar's mother. In the book, it states that there are rumors that she is a witch. She is most likely the conspirator of Finn's "death".
She is said to appear as a young, beautiful woman, but she only creates that appearance with trickery and is quite old.
She is a selfish woman who will stop at nothing to keep herself in power.