Shields Green,Fugitive slave from Charleston, S. C. Joined Brown at Chambersburg, having come there with Frederick Douglass, August 19th; was known as the "Emperor," but how he obtained this name is not now known. He went on with Brown when Douglass turned back, telling his former benefactor "I believe I'll go with the old man." Green's age is said to have been twenty-three years.

Source:

http://www2.iath.virginia.edu/jbrown/men.html