Charlemagne is a pimp and drug dealer who is often mentioned in Hold Steady songs. Holly mistakes him for her savior.
Because Charlemagne is dealing drugs (and given the many popculture and musical references in Hold Steady songs), it is possible that the name is meant to be a reference to the Steely Dan song "Kid Charlemagne" which is loosely based on the LSD-chemist Augustus Owsley Stanley III.