Born the youngest child of Lord and Lady Dunsany, Isobel was raised alongside her older brother, Gordon, and older sister, Geneva. Gordon was killed in the Battle of Prestonpans during the Jacobite Rising of 1745; the Dunsanys took the loss very hard, and perhaps as a consequence cherished and spoiled their daughters even more than before.
- See also: Voyager
In 1768, Isobel died of a "bloody flux" aboard a ship to Jamaica, where she and William intended to join Lord John, who was governor of the island at the time.
In 1758, Isobel grieves the loss of her sister, insisting to Lord John Grey that she doesn't know how to go on living. Lord John advises that it helps to break things, and promises to shoot clay pigeons with Isobel before he returns to London.
In 1760, Jamie Fraser, a groom on the Dunsanys estate, transports Isobel to the village to see a Mr. Wilberforce. Jamie witnesses Isobel in a passionate embrace with the gentleman in his garden, but does not interfere.
Months later, when Jamie has returned from London, Isobel's maid Betty Mitchell implores Jamie to help retrieve Isobel, who has eloped with Mr. Wilberforce. Knowing that Isobel's reputation will be ruined if anyone discovers what has happened, Jamie rides through the night to find her and returns her to Helwater. Jamie deposits her with Lord John, trusting in his discretion and knowing he will take care of her.
Described as small, round, bubbly and blond, with pale bluish-green eyes like her mother.
- Lord John Grey – Isobel marries Lord John around 1764, and they effectively become parents to her sister's son, William.
- Isobel is the Scottish form of the name Isabel, the medieval Occitan form of Elizabeth. Ultimately derives from the Hebrew name אֱלִישֶׁבַע (Elisheva; English Elizabeth) and means "my God is an oath" or "my God is abundance".
- Dunsany is the name of a title in the Peerage of Ireland, as well as a castle in County Meath. It comes from the Irish Dún Samhnaí.