The Mirror Dragon is the secondary focus of the two books, the primary focus being Eona.
The Mirror Dragon is one of the twelve celestial dragons. However, the Mirror Dragon is actually the queen of all dragons, meaning she is female, explaining one reason why Eona was selected as the Mirror Dragoneye. Due to a deference shown by the other eleven dragons during her return, she shares ascendency with the Rat Dragon on his year. However the Mirror Dragon has been lost from the circle of dragons for over 500 years for reasons unknown.
It is the keeper of one of the greater virtues, Truth, and one of the twelve celestial directions, East.
We also find out that the previous Mirror Dragoneye, Kinra, was seen as a traitor by the imperial throne and therefore executed after attempting to steal the Imperial Pearl. But, what she really wanted to do is to take the imperial pearl and save the dragons diminishing power by setting them free of their enslavement by mankind.