Thomasina Coverly is the young daughter of Lord and Lady Croom. She is 13 years old at the start of the play, an age symbolic of her position on the threshold between childhood and adulthood. Even later, at 16, she has a youthful innocence and imagination. Unlike her mirror character (Hannah), Thomasina lacks sexual knowledge and experience. Thomasina should, according to her mother, be ignorant like “an empty vessel waiting to be filled at the well of truth—not a cabinet of vulgar curios” (11). In sharp contrast, Thomasina has advanced mathematical knowledge. Her genius is ahead of her time because she is able to ask questions and make connections that adults cannot. Although Thomasina represents Enlightenment commitment to rationality, her curiosity about sexuality and her romantic inclinations toward Lord Byron reflect the intertwining of the rational and the more Romantic in her character.

Her tragic early death mirrors the description of the Library of Alexandria. Thomasina dies in a fire, which also destroys the library. Despite this great loss, later scholars rediscovered the lost work of the library: Septimus expresses this idea when he describes how “[m]athematical discoveries glimpsed and lost to view will have their time again” (38, emphasis added).