Lady Isabella Fitzwilliam is a character from Season Two of Harlots, and is played by Liv Tyler.
Lady Isabella Fitzwilliam is a woman of exceptional wealth and grace who lives an enviable life of leisure. Yet hers is a gilded cage, her illegitimacy and sex leaving her at the mercy of the whims of her charming but calculating brother, the Marquess of Blayne. She depends on him for her every need, and he restricts access to the money which might buy her some independence or power. And Lady Fitz must keep him sweet for she has secrets which threaten everything she holds dear and keep her in the thrall of Lydia Quigley. Yet she will find an unexpected friend and ally in courtesan Charlotte Wells.
Lady Isabella Fitzwilliam is women of high social standing who appears to have it all, however her life is not as rosy as it may seem. She lives in a stately manor with her brother Lord Harcourt Fitzwilliam, who seems to have much control over his sister. She is unable to access her own funds left to her by their father, as Harcourt refuses to give her control over her own inheritance. Despite her beauty and wealth, Lady Isabella is unmarried, something which confuses those around her. She harbors a dark secret. Some years before she is introduced to the series, Lady Isabella was raped by her brother Harcourt. This rape resulted in a child, who was born while Harcourt was away, giving Isabella the opportunity to hide her daughter in a school. Lady Isabella is frightened for her daughter's safety, and because of this only sends one letter, telling Sophia they should never meet.
Lady Isabella Fitzwilliam is introduced in season two when Charlotte is sent to her on the orders of an imprisoned Lydia Quigley. Quigley is aware of Lady Fitz's dark secret, and had apparently been blackmailing her for some time. Charlotte and Lydia extort the 500 pound fine keeping Lydia in jail from Lady Isabella, who must lie to her brother, telling him its to pay a gambling debt to Charlotte.
Charlotte is intrigued by Lady Fitz, and the women strike up an unusual friendship.