Who is Luke Gopnik from Paris to the Moon and what is their importance?

Luke Auden Gopnik, the son of Adam and Martha, was an infant when the family moved to Paris and was five years old when they returned to New York City. In the first part of the book, his role was principally as an inspiration for his parents to move to Paris, and as a focus of their hopes for him in those early years of his life. As Luke began to walk and talk, Gopnik reported a few amusing or surprising statements he made, as proud parents tend to do, and his early, hesitant forays into the big world. For much of the book, Luke was too little to have developed much of a discernable personality, but he seemed to have a mixture of self-confidence and timidity, as if he knew his limits.