Who is David Henry McCallister from The Memory Keeper's Daughter and what is their importance?

Having been raised in poverty, David Henry has made the decision to have a better life than his parents were able to provide. As a young child, he watched his younger sister, June, suffer from a heart condition, and then watched his mother's inconsolable grief when June died. His compassion and his wish to fix things for people resulted in his decision to become a doctor, however, he chose osteopathy because of the solid, predictable nature of human bones.