Who is The Daughters of Glass from Key of Valor and what is their importance?

The legend goes that the King's daughters were three demigoddesses, daughters of a wayward god king and his human wife. When the king separated from his father's kingdom, the world split into two pieces. The second piece was ruled by an evil sorcerer named Kane.

"The young king had three daughters. They call them demigoddesses because they're part human. Each of the daughters had a special gift. One was music, or art, another was writing, or knowledge, and the third was courage, I guess. Valor." Chap. 1, p. 9

The demigoddesses were guarded by a warrior and a teacher, who subsequently fell in love and neglected their duties. Kane cast a spell on the demigoddesses, locking their souls in three glass boxes. The only way for them to be released is if three mortals, the demigoddesses counterparts, could solve a riddle, find the right key and open the box. Zoe's counterpart is Kyna; Malory's counterpart is Venora; and Dana's counterpart is Niniane. Key of Valor is the final book in the trilogy and focuses on Zoe/Kyna's mission.