He is a gangster who lived in the same building as Angus Fangus back in New Zealand, and possibly his only friend. He has a violin case where he actually keeps a violin, but plays it so bad that Angus thinks a machine gun would be less murderous.
Despite being a gangster, Billy isn't portrayed as the villain in the comics, and actually seems more honest and sensitive than Angus. However, his gangster side is sometimes shown when one of his hits coincides with Angus' scoop, or when he finds members of a rival gang.
Since all "Angus Tales" are colored in a sepia tone, the actual colors of Billy's persona and clothes are unknown.