Who is Felix Leiter from Live and Let Die and what is their importance?

Felix Leiter is an American CIA agent. He has worked with Bond in the past at the Casino Royale job. They get along well and are partners again in this novel. While in Florida Leiter goes after The Robber and is put into a shark tank. The shark nearly kills him and he loses one arm and one leg. His face is also seriously injured. At the end of the novel it is revealed that Leiter will live.