Wikipedia told me this.
is the national personification of the Southern states of the United States. The latter part of his name is derived from Rebellion. Patriots used Johnny Reb and his Union counterpart Billy Yank to symbolize the lost souls in the American Civil War of the 1860s.
Johnny Reb is usually pictured in gray wool uniform that included the confederate flag. Johnny Reb was typically a young poor uneducated white male who had not yet attained 15 years of age.
Definitions.net told me this:
Johnny' was applied as a nickname for Confederate soldiers by the Federal soldiers in the American Civil War; `greyback' derived from their grey Confederate uniforms