Our speculation was true. It's confirmed that the silver hair girl on the box art is in fact the princess of the empire, and is voiced by Nana Mizuki (Japan's highest-earning singer voice actress). She's songstress of the enemy side. Inevitably, she's Kirika's rival and will most likely have her own song.
Additionally, Kizuna Diagram system is revealed. From the pictures, it seems like you can set a personality to a character which will affect relationship between party members and can trigger an event.
Shining Resonance will be released on December 11. Sonia is expected to be playable in Beast Note's most anticipated fighting game Blade Arcus.