Billy Yank is the personification of the Northern states of the United States, or less generally, the Union during the American Civil War. The latter part of his name is derived from yankee, a slang term for Americans. Political cartoonists used Billy Yank and his Confederate counterpart Johnny Rebel to symbolize the combatants in the American Civil War of the 1860s.
Billy Yank is usually pictured wearing a wool uniform that included the fatigue blouse, a light-weight wool coat with an inside pocket and four brass buttons on the front, with a forage cap made of wool broadcloth with a rounded, flat top, cotton lining, and leather visor. This is indicative of the fact that the Union uniform was standardized by U.S. War Department.
Billy Yank in publications owned by DC Comics is tied to that of Uncle Sam in that company's comics. In this version, the "Spirit of America" (who is currently embodied as Uncle Sam) had taken human form as the Minuteman, Brother Jonathan and, during the American Civil War, had been split in two as Johnny Reb and Billy Yank.