Who is John Claggart, Master-at-arms from Great Short Works of Herman Melville and what is their importance?

Claggart is a petty officer, the master-at-arms of Bellipotent; the title is a little misleading, as discussed in Chapter 8, because Claggart's official role aboard ship is to maintain order, somewhat like a police constable—he does not instruct anyone in the use of arms. Because of his role, Claggart is of necessity quite unpopular with the crew, who refer to him as Jimmy Legs.