James Dillon was a young Royal Navy officer. After serving as lieutenant on the HMS Burford, he is appointed to HMS Sophie as lieutenant in place of the previous lieutenant, Mr Baldick, following Jack Aubrey's appointment as Commander. Jacks finds dificulty ocating him at first, but they all set sail and capture many prizes while excorting convoys. While Jack was pleased with his First officer, Dillon's relationship with Aubrey is always complicated, and often fraught. Dillon believes that Aubrey is overly interested in prize money above duty, and that he is "shy" - another way of describing cowardice. His relationship with Stephen Maturin, is far more congenial, initially because they were previously acquainted, both being members of the United Irishmen. They share an Irish heritage, and also the Roman Catholic faith. He was wounded and killed during the action with the Cacafuego, leading a boarding-party.