Guardian Of The Spirit by Nahoko Uehashi

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.

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