Captain Peculiar Cromwell was the commander of the British ship Calliope. His parents were devout Christians who chose his name from a phrase in Deuteronomy and sent him to the sea to keep him away from temptation, leaving him to see to his own education. He served in the Royal Navy but realised there was more profit as a civilian captain so took a post with the East India Company. He occupied a modest cabin aboard the ship so he could sell the largest cabins to passengers.
In 1805, Cromwell was assigned to transport a party including Ensign Richard Sharpe to England. Prior to departure, he made a deal with Anthony Pohlmann, who was a passenger under the assumed identity of the Baron von Dornberg, and French agent Michel Vaillard, who was posing as "the Baron"'s servant. Cromwell believed the British were incapable of raising an army large enough to defeat the French, and would either be invaded or forced to make peace, so saw his priority as profiting from the war. He agreed to allow his ship to be captured by the French in exchange for a share of the prize money, to which aim he deliberately steered the ship away from the Company convoy. He aroused suspicion by treating Pohlmann as a stranger when he dined with the passengers despite their many meetings in private.
Cromwell noticed Sharpe was carrying the Tippoo Sultan's jewels sewn into his coat and convinced him to hand them over for safekeeping, making similar demands of the valuables of Lord William Hale and Major Arthur Dalton, and also attempting the same of the merchant Ebenezer Fairley who refused. When the French ship Revenant was sighted, Cromwell delayed turning away from it and hampered the Calliope's flight with constant course changes. He eventually surrendered to Captain Louis Montmorin as arranged and transferred over to the Revenant with Pohlmann and Villard, taking the passengers' valuables with him along with his own belongings.
After the Revenant was captured by the HMS Pucelle at Trafalgar, Cromwell again encountered Sharpe and noted he had told Montmorin to avoid the fight and continue to Cadiz. Sharpe told Pucelle crewman Clouter to take Cromwell to the hold and retrieve his and Dalton's belongings and that Clouter could keep anything else he found. Captain Joel Chase insisted Cromwell be kept alive in order to stand trial.
He appeared in Sharpe's Trafalgar.