Stephen Maturin is one of the two main protagonists of Patrick O'Brian's Master and Commander novels. He's a ship's surgeon on various vessels under the command of Captain "Lucky" Jack Aubrey, the books' other hero. Stephen's secondary employment, which sometimes abets but often hinders his duties aboard ship, is gathering intelligence and serving as a general factotum for Britain's intelligence services.
He also had a love thing going with Diana, the cousin of Jack's wife Sophie.