Septimus must make his living and thus is tutoring the daughter of Lord and Lady Croom on their estate. At the same time, he writes reviews of the latest literature but seems to take neither task very seriously. A dashing playboy too smart for what he is doing, he amuses himself with flirtations, if not outright affairs, with the women on the estate, including Mrs. Chater and Lady Croom. He is a close friend of Lord Byron, a classmate at Cambridge. Seemingly devoid of emotional attachment or sincerity at the beginning of the play, he manipulates less intelligent characters with his witty verbal gymnastics. He later softens and develops feelings for his student Thomasina and finally becomes a hermit, crazed by trying to solve her equations—which form the basis for chaos theory—after holding himself partially responsible for her death because his restrained sensibilities would not allow him to seduce her.
The 13-year-old daughter of Lord and Lady Croom, Thomasina displays brilliant insights into mathematical theory—iterative algorithms—200 years ahead of her time. Her seemingly innocent observations about the world and her immediate situation often reveal profound truths and an astute wisdom greater than the adults around her. At 16 she develops romantic feelings for her tutor, Septimus Hodge, but dies in a fire soon after.
The foolish, dim-witted husband of the lustful Mrs. Chater, Ezra Chater seems oblivious to his wife's true nature. In his desire to be accepted and admired, he is easily flattered and manipulated by the people around him. He twice challenges Septimus Hodge, who had a liaison with his wife and ridiculed his poetry, to duels that never happen. After leaving Sidley Park in disgrace, he travels to the West Indies with his wife and her lover, Captain Brice, where the captain establishes him as a botanist; he actually discovers a small dahlia and dies the next year from a monkey bite.
Although her first book on Lady Caroline Lamb, a writer and mistress of Lord Byron, became a popular success, Hannah Jarvis is a serious scholar who displays animosity toward strictly academic scholars, particularly Byron experts who panned her last book. She is a no-nonsense woman with no patience for flattery or lies. She rejects Bernard Nightingale's sexual advances and values her work more than emotional and physical involvement. She is writing a book about the hermit who lived on the grounds of Sidley Park and has been reviewing the estate records as part of her research. Both Gus and Valentine Coverly have crushes on her.
A condescending know-it-all, Bernard Nightingale feels compelled to show off his knowledge whenever possible, especially if it is something he thinks someone else does not know. He treats Hannah as a student rather than a peer, repeatedly telling her the terms for what she is describing. Chloë Coverly's nickname for him, "Peacock," is an apt description of his haughty, preening manner. He is a superficial scholar, a man in love with the sound of his own voice, his own ideas, and his own hasty and ill-founded conclusions; he is unwilling to hear the truth unless it is convenient to his desires and is horrified when his hasty conclusion, already made public, is proved false.
A self-deprecating University of Oxford graduate student with a dry wit, Valentine Coverly is researching the mathematics of chaotic systems. He is attempting to use estate records in his research but becomes frustrated by the number of variables. He seems infatuated with Hannah and astounded by Thomasina's genius, far greater than his own intelligence given that hers was unaided by computers.
With her forceful personality and strong opinions, Lady Croom is used to having people agree with her. She opposes the changes her husband has planned for the estate gardens, which she likes as they are in their ordered classical designs as opposed to the newer, wilder, and more Gothic-oriented plan. She enjoys the romantic attention of the young scholars invited to the house: first Septimus and then Lord Byron.