Diana Villiers is Sophie's beautiful cousin and her opposite in many respects. She has a great love of horses and riding and breeds Arabians. Her impetuous nature has led to more than one ill-conceived affair, but eventually Maturin wins her over. Diana is a black haired woman with blue eyes, strikingly tall and slender. Her appearance is of some importance to her and she often capitalizes upon it. She is a woman with a reputation and is perceived to be flirtatious. Both Jack Aubrey and Stephen Maturin fall for her; indeed she is a woman who is attractive to many men. Diana temporarily flees her home after the birth of her daughter Brigid, unable to deal with the young girl's autism.
Diana Villiers is the daughter of Mrs. Williams's sister, and of age with Sophie. Though English by birth, she has spent much of her life abroad, particularly in India, where both her father and her first husband were officers in the British army. She also remembers a stay in Paris as a girl.
She returns to England, while there Mrs Williams views Diana as a threat to Sophie's marriage prospects, and first bribes her with her own horse, and later arranges for her to care for their mad Cousin Edward. Stephen, although with no inducement from Diana, falls in love with her. Jack, though fond of Sophie, finds little encouragement when Mrs. Williams discovers the state of his finances. He then turns with more affection to Diana, to Sophie's jealousy. Eventually, the two women have an argument, and Diana leaves the Williams household permanently.
While Jack is stationed in The Downs, Diana pursues a romantic relationship with him, and insists on pure friendship with Stephen. This leads to the men's estrangement and nearly a duel. Eventually, she goes into keeping with Richard Canning, a wealthy married Jewish businessman in the East India Company, and the couple travel to India.
Jack, now engaged to Sophie, brings HMS Surprise to Bombay, where Stephen plans to meet Diana. She is unhappy with her situation, and thus encouraged, Stephen proposes. Upon his visiting her again in Calcutta for her answer, they are interrupted by Canning, which results in a duel. Canning is killed, whether on purpose or accident is uncertain, after he severely wounded Stephen. While Stephen is convalescing, she accepts his proposal and agrees to sail for England in the East Indiaman Lushington, after Jack first refuses her passage in the Surprise. The Surprise leaves later than the "Lushington," and when he reaches Madeira, Stephen learns that Diana has left with Harry Johnstone, for America, in hopes of making her fortune. She leaves a letter, with Stephen's ring, in Madeira.
She returns to London, and is once again pursued by Stephen. However under the accusation of spying (for America) she flees again to Boston under the protection of Johnstone. Among the friends she leaves behind is American Louisa Wogan, who is sentenced to be transported in HMS Leopard for that very crime.
While in America she becomes pregnant with his child. After many months of living with the man, she has become an American citizen, and disgusted with Johnson, when she learns that he is a American spy, and the cause of the accusations of spying against her. Jack and Stephen once again are brought into her life when they are brought to Boston as prisoners of war. Stephen takes refuge in her hotel room while on the run from a party of French spies, she hides him and helps him to secure Johnson's secret papers; however, she still resists his renewed proposal of marriage. Jack escapes with Stephen and smuggles Diana away with him.
At sea on a small boat with Aubrey steering, they soon find the HMS Shannon, blockading the harbour. They are taken aboard HMS Shannon and are present throughout her battle with the USS Chesapeake. She is labouring selflessly among the men wounded in the battle. Diana, traveling with Stephen (her lover) and Jack on a packet back to England, are pursed by privateers, sent by Johnson. One sinks and the other gives up, and upon reaching England she travels to Paris, to stay with a friend Adhemar de La Mothe for her lying-in. The baby is unfortunately lost.
Diana learns that Stephen, and Jack are being held in Paris, and after meeting with Stephen, gives her great blue diamond, to a Minister's wife to help secure their release. She escapes with Jack and Stephen, and a young Swedish naval officer named Jagiello. They run to a waiting cartel at Calais, the Oedipus commanded by William Babbington. Safely away, Stephen proposes to Diana Villiers once again and they are finally married on board by Babbington.
Their marriage is an odd one, but works out until rumors of Stephen having an affair with a girl from Malta during a voyage under Aubrey's command. When no reply is sent to explain if they were true, Diana accompanied by Jagiello go to Sweden. Jagiello believes she wanted to have a relationship, but she tells him that she didn't, and helps him find a wife. She buys a hot air balloon during her stay. Soon after she learns of the distressing and horrible new of Aubrey being struck off the Navy list, causing anger towards the Admiralty, Stephen comes while waiting for Jack to pick him up in the Surprise bound for the Americas, Stephen finds Diana and she is overjoyed, and forgives him, but saying she will not marry anyone ever again. Even after Stephen explains that the rumors about him at Malta were untrue. After Stephen is injured, she changes her mind and they once again marry.
While Maturin is on another voyage to the Far East, Diana gives birth to a daughter, Brigid, who appears to suffer from a form of autism. In despair, Diana leaves Brigid and disappears. However, in The Commodore, Maturin tracks her down again in Ireland. They are reconciled again as Brigid makes a full recovery.
Diana and Brigid spend a lot of time with Aubrey's family. Diana enjoys driving a carriage and team of horses. Her death arrives during the One Hundred days, when she dies in a carriage accident.
Diana is killed, along with her aunt Mrs. Williams, when the carriage she is driving goes out of control and overturns. This is believed to be a reference to O'Brian's own wife's death.
Stephen is actually searching the hotel room in Boston during which search he kills two french agents
before Diana enters