Who is Natalia's Grandfather (Dr. Leandro) from The Tiger's Wife: A Novel and what is their importance?

Natalia's grandfather is portrayed as strong willed, independent, determined, and a fighter. He wants to live, and he wants other people to live, and apparently fights until his very last moments to ensure this happens. Each of the book's three main narrative lines chronicles and explores aspects of his life during a different phase of that life (each, by the way, anchored to one degree or another by his determination to live himself and help others to do so).