Half-Catalan, half-Irish physician, surgeon on board most of the vessels commanded by Jack Aubrey, and Aubrey's closest friend - and, briefly, rival. Owns estates in Catalonia, and is employed as a secret agent by the Admiralty, occasionally taking him on land missions in which Aubrey does not take part.
Plays cello to Aubrey's violin, and is what the eighteenth century meant by a philosopher - someone with wide-ranging scientific interests, though primarily in botany and zoology, as well as medicine.
Is regarded almost superstitiously by the crews with which he serves, especially after he trepans the skull of a gunner who has been hit on the head and replaces the cut-away skull with a metal plate, relieving the pressure on his brain. He also "brings back to life" a midshipman who appears to be drowned.
Somewhat unbelievably, he remains throughout the series virtually totally ignorant about Navy customs, jargon and attitudes: his lack of skill at such things as climbing rigging and coming aboard means he is treated by the crew as a kind of respected pet. Says he has a horror of appearing eccentric, but manages to do so throughout: in Post Captain he rides a mule to a hunt. Can be roughly approximated by imagining Dr. Doolittle - or possibly Dr. Who - has shipped in a man of war. He is however a deadly marksman with pistols and a skilled swordsman and card player.
In 'Post Captain' falls in love with Diana Villiers, a less than reputable widow whom he eventually marries, though she also has an affair with Aubrey (and others). She is cousin to Aubrey's wife Sophie, and the occasionally tangled relationships that emerge provide a continuing counterpoint to the naval adventures.
As an Irishman, Maturin was a member of the United Irishmen, who rebelled in under Wolf Tone. As a Catalan, he works for Catalan independence from Spain, work that overlaps with his work as a secret agent for the Admiralty.