Who is Nate Warren from Ella Minnow Pea: A Progressively Lipogrammatic Epistolary Fable and what is their importance?

Nate Warren travels to Nollop posing as an "old family friend" of the Purcy's. In reality, he is an American researcher investigating the situation in Nollop. Over the course of the story, he falls in love with Tassie, eventually rescuing both Tassie and her mother. In the end, Tassie and her mother move to America with Nate.