Captain Justice is an agent for the British Naval Intelligence.
That is what one source indicated but two other sources show that Justice, full name John Valcourt Justice, is a naval officer who is seconded to shore duty to perform missions for the very mysterious clandestine organization with the odd name of Board of Beacons, Bells, Buoys and Mercantile Messengers, which I have shortened to The Board.
The period of time that Justice operates is 1804-1807. Napoleon Bonaparte has been in control of France for a handful of years since seizing power in 1799 and has just crowned himself Emperor. His ambitions for control of Europe are well known and when we first encounter Justice, he and the rest of the British Navy are preparing for war in the defense of England.
At times Justice has been given command of a naval vessel but his usual status is to be without a command, something that brings "a pang of genuine regret". for "there were time when he longed to walk his own quarter-deck again, but once a man was in the service of the [Board], it was goodbye to hopes of that sort".
Justice is described as a "trim, sandy-haired man in the full fig of a captain [of the] Royal Navy". Elsewhere he is said to be a handsome fellow, self-reliant and strong".
A deeply patriotic man, which explains why he agreed to give up a career at sea which he loves for the clandestine work that the Board asks of him, Justice nevertheless often has little good to think about those in control of the country, politically or militarily, damning those "in their cups who would blab away the fate of an army at the dinner table". He similarly has little good to say about "Whig journalists who told a war-weary nation what it wanted to hear, not what it should hear".