Zandria receives the "call" in the form of the werewolf attack. Up to this point, the stage has been set for a grand adventure, but Zandria and Olena only know their tiny little world on the pristine beach of Banookanook.
Joseph Campbell states that an inciting incident needs to occur to get the story going. Something needs to create disorientation and discomfort for the hero to force them to move out of the "normal world" to the special one. This is quite often a major loss in the character's life. Luke Skywalker loses his aunt & uncle, Zandria loses her father. Then the character has no other option but to follow the call.
In Zandria's case, if she does not choose the path of adventure, her story is ended. She simply can not stay with the werewolves. She is literally forced into her own hero's journey.
Of course, she does not go alone. She is accompanied by her sister and Kez, the monkey-like Quzzak. Because they are sisters, they are like two halves of the same whole. They both share this journey and Kez serves as their mentor, and also a little comic relief. Once the girls meet William, Kez's role recedes a bit because William is more experienced in the "special world". Kez is actually a reluctant member on the journey as well. He did not really want to go, but his love for the girls forced him.
The Empyrical Tales are the stories of both sisters, but The Fourth Queen is ultimately Zandria's journey. The majority of the story is told from her POV and we travel with her through each stage. Olena is also a hero, but she does not get her own journey until The Secret Queen.