Who is Long John Brown from The Complete Poetry and Prose of William Blake and what is their importance?

Long John Brown appears in Songs & Ballads, in which, he is tormented by the Devil because of his love for another. In the poems, we learn little about Brown outside of the fact that he was overthrown by the devil over a love difficulty. As a result, Brown was left with an extremely fast metabolism and enormous appetite. Thus.... he ate and drank to excess but withered away to nothingness. In the end, we learn that his problem wasn't caused by himself or Mary, but rather by the manipulations of a fairy and the devil's own opportunism.