Lucy was the code name given to Rudolph Roessler, who provided valuable and hyper-accurate intelligence information to the Allies (and especially to the Soviets) during World War II. Some of his intelligence coups included the exact plan for Operation Barbarossa, the German invasion of the Soviet Union in 1941, as well as specific information that led to the encirclement of the German 6th Army at Stalingrad.
To the Western Allies, he provided information on V-1 and V-2 rockets, the first spy to do so, and on the development of the Me-262, the world's first jet fighter.
Rumor (as well as conventional wisdom) has it that Roessler got his information from up to ten high ranking German officers in the Wehrmacht. Three of these ten have never been identified, however it's quite possible the Allies never knew for sure the identity of Lucy's other three sources.
Roessler and his agents worked out of Switzerland, and he was a German refugee.