The Chrysalis Queen Quartet, book 1: Guinevere's GiftNancy McKenzie2008

On the night of Guinevere’s birth, there was a prophecy that foretold she would one day be highest lady in the land and wed to a great king. But 13 years have passed, and the prophecy couldn’t be further off. Guinevere is now an orphan and a ward of her aunt and uncle, the king and queen of Gwynedd. Tomboyish and awkward, Gwen is no great beauty, and nobody takes the prophecy seriously–especially not Gwen.
But then one day Gwen meets a strange young man in the woods who claims to be part of an ancient tribe whose mission is to guard and protect her. Then she stumbles across a sinister plot brewing within the castle walls–one she alone might be able to prevent. Guinevere is beginning to realize her destiny is more complex than it seems–and this is only the beginning.