Who is The Rookie from Paris to the Moon and what is their importance?

The Rookie was an imaginary little boy concocted by Gopnik in a bedtime story for Luke that went on, with embellishments, for two years. The Rookie, who had the gift of throwing stones with incredible speed and accuracy, got a tryout as a three-year-old with a professional baseball team and became a star pitcher. When Gopnik wrapped up the story by having the Rookie run home to his mother, Luke protested so fiercely that the Rookie had to be recalled to the team.