Who is Mazer Rackham from Shadow of the Giant and what is their importance?

Mazer Rackham is a famous war hero who saved the human race from annihilation during the Second Invasion. He's a bit of a mystery. Everyone thought he died in the war, but in reality he had been sent away at near-lightspeed to keep him alive to be available for the last battles of the war. Han Tzu met him briefly at Command School on Eros, when he trained their commander, Ender Wiggin, for the war against the Hive Queens.