The last altruist

It was a sunny afternoon following a very stormy evening. A fisherman notices a small empty boat on the calm waters of the river. He approached it and discovered a young girl sleeping peacefully under many algae. Despite waking up in a stranger's house, she remains calm and serene, a smile even appears on her face when she sees the sleeping dog at the foot of the bed. Her first words, "Thank you for your help, I promise to make your dream come true". Amazing words from a 5-year-old girl. Who is she and where does she come from? What is this dream?