Aqua Jackson is Percy's younger sister by two years. They have a normal childhood until they learn that they are the children of Poseiden.
Now they live at Half-blood camp. A camp for fellow demigods. Its safe there.
But they must soon leave the comfort of their camp to face the hardships of the world. Because Zeus's lightning bolt has been stolen and he blames them.
They must retrieve it or face an all out war between Poseiden and Zeus.
-Follows book. Not movie!