Louisa de Clermont was Ysabeau de Clermont's vampiric daughter and sister to Matthew de Clermont. Like Matthew, she suffered from blood rage. She was killed in Barbados, when local plantation owners seized the opportunity of a rebellion to cover up their deed. Her brother described her as absorbing every vice of every age she lived through.
A Discovery of Witches
While Louisa herself does not appear in the book, Diana Bishop notices her portrait hanging in the gatehouse of the Old Lodge. Matthew Clairmont tells her that Louisa went to Barbados intent on making herself "queen of the Indies", despite warnings that her taste for young men would be noticed on a small island.
Shadow of Night
When Matthew and Diana visit Sept Tours in 1590, Diana uses Louisa's room until their marriage ceremony. Phillipe de Clermont seems unsure of where Louisa is at the time, "Vienna or Venice" and complains that he cannot keep up with Louisa and her wanderings.
After Matthew and Diana return to London from Prague in 1591, they must report on their journey to Queen Elizabeth at the Palace of Greenwich. Unbeknown to them, Louisa has arrived in London, and Christopher Marlowe has been reporting to her his suspicions about the witch who has entangled Matthew.
Marlowe lures Diana into one of the tiltyards, where he and Louisa use her as a jousting target. Both are under the influence of drugs, and fortunately are either unable or unwilling to kill Diana immediately. Louisa states that Diana does not deserve a quick death. Diana is able to free herself, and furiously predicts their future, telling them death is coming for them and it will not be gentle.
When Hancock arrives to help Diana, Louisa reveals to him that she believes Diana to be the witch from the prophecy that Gerbert of Aurillac shared with Ysabeau de Clermont, and will bring down the de Clermonts. While neither Diana nor her unborn children are seriously injured, Matthew is furious.
Both Louisa and Marlowe are imprisoned in Bedlam by Matthew, who feeds off both of them. When Diana arrives with Father Hubbard to intervene, Louisa is not in her right mind. She has been chained and is being tortured by a stag's head dripping blood just beyond her reach. She is disappointed when Diana will not use magic to erase her memory of the events at Greenwich, and describes blood rage as having "these things crawling in your skull".
When Diana asks that the family make sure Louisa is not a danger to anyone before they set her free, Gallowglass de Clermont tells her Phillipe de Clermont will never let her roam again. After her actions in London, Phillipe sends her to what is later described as "the far reaches of the known world".
The Book of Life
Since Louisa's actions in London were not public knowledge outside of the small circle of de Clermonts who already knew of Diana's timewalking abilities, it is only when Diana and Matthew return to Sept Tours that Phillipe's actions in disciplining Louisa and his lack of response to her death make sense to others. Fernando remarks that Phillipe "hadn't stirred a finger" to avenge her.