Michael Christopher Hunter's real name is Gerhard Wagner. He is Alex Hunter's grandfather.
Wagner was a German soldier, who worked as a radio operator in the U-boat headquarters during WWII. Since Alex was a little girl, he told her a story of how the Nazis had collected the most beautiful things in the world and put them in a submarine. When the ship was crossing the Atlantic Ocean, it went down and no one could find it. Wagner had received the final SOS signal from the submarine and encoded the SOS antenna design into a Music Box. His plan was to go to America, build a receiver and find the U-boat. (Under the Radar)
After the fall of the Third Reich, he did flee to America. However, he lost the music box. In America, he claimed to have top-secret information he wanted to share with the U.S., but was sent to a detention center at Ellis Island. He then escaped from the detention center a few weeks later, disappearing with his information. He then changed his name to Michael Hunter and became a U.S. citizen (immigrated through Ellis Island in 1946). (Power Play)
Wagner died on June 5, 2000 at Coney Island Hospital in Brooklyn, New York. He was preceded in death by his wife, Sadie Hunter. On his deathbed, he told Alex that the story was true, and proved it by giving her a piece of the music box that he had kept. The search for the music box was thus passed to Alex. (Under the Radar, Power Play)