Seirei no Moribito

From Scholastic: Balsa was a wanderer and warrior for hire. Then she rescued a boy flung into a raging river - and at that moment, her destiny changed. Now Balsa must protect the boy - the Prince Chagum - on his quest to deliver the great egg of the water spirit to its source in the sea. As they travel across the land of Yogo and discover the truth about the spirit, they find themselves hunted by two deadly enemies: the egg-eating monster Rarunga and the prince's own father. A 10-book series with multiple side stories and a cookbook.

Once every 100 years, a Water Fairy implants an egg into a person who then becomes the Guardian of Fairies. This time, the responsibility falls on Chagum, the second prince of the New Yogo Kingdom. Chagum\`s life isn\`t easy. The king finds Chagum an inconvenience and attempts to have him killed. As a result, the queen asks a female guard, Balsa, to protect Chagum. In addition to this, beings from another world, known as Nayugu, constantly attack him in order to eat the egg carried in Chagumu\`s body.