The Lost Princess Series - First Book:
Katharina, she's the last member of the royal family of Narnia. When The White Witch, Jadis invaded to Narnia, Katharina's Godfather, Aslan, send The five years old Katharina away from Narnia for her own protection.
She was sent to the old professor Kirke, who took her in right away. Katharina was told that one day she will return to Narnia when the time will be right.
The years go by and Katharina started to lose hope of returning to Narnia and her family. But it's changed when the Pevensie kids come and stay with the Professor, and Katharina started to feel hope once again.