- Thomasina is the young, impetuous genius child of Lord and Lady Croom aged thirteen and later sixteen. Thomasina miraculously theorizes the second law of thermodynamics and understands chaos theory way ahead of her time.
in-depth analysis of Thomasina Coverly.
- Academic and tutor of Thomasina Coverly, Septimus works on his own research while teaching Thomasina. Septimus falls in love with Thomasina and, after her death, spends his time researching and attempting to prove her theories.
- Jellaby is the distinguished, middle-aged butler of the Sidley Park who delivers many letters.
- Ezra Chater is a poet and amateur biologist-neither of which he does particularly well. Mr. Chater's wife is constantly cheating on him and has made her rounds between almost all of the men at the estate. Mr. Chater dies in Malta of a spider bite.
- Mr. Noakes is the gardener who excels at frustrating Lady Croom with his new garden plans, spying on carnal embrace, drawing beautiful pictures of the estate and building an improved steam engine. Noakes has drawn up plans to transform Sidley Park into a classical masterpiece.
- Lady Croom is the bossy, battle-ax who storms around the estate. There exists two Lady Croom's in Arcadia
; however, we only meet one—the Lady Croom of the early nineteenth century. Lady Croom is perceptive of all things and knows all happening on the property.
- Captain Brice is the sea captain, apparently in love with Mrs. Chater. Brice takes Mr. and Mrs. Chater on his sea voyage to Malta, where Mr. Chater perishes.
- Hannah is the champion of academic knowledge in Arcadia.
While her discoveries are less important than young Thomasina's, Hannah has all but rejected any notion of romantic knowledge in favor of intellectual work. Hannah is an author working on the hermit of Sidley Park and is in her late thirties.
in-depth analysis of Hannah Jarvis.
- Chloe Coverly is Thomasina's modern day equivalent. Chloe, aged eighteen, seems as perceptive as Thomasina, but lacks the education to have Thomasina's brilliance. Chloe argues that a Newtonian universe would be destroyed by the random nature of sexual acts.
- Bernard is the modern fool and fop. In his late thirties, Bernard leaps over truth in favor of finding fame for his theory that Lord Byron killed Ezra Chater. The theory, eventually proved false by Hannah Jarvis, brings Bernard great shame and embarrassment.
in-depth analysis of Bernard Nightingale.
- Valentine, aged twenty-five to thirty, is still supposedly a graduate student studying mathematics. The son of the Coverly estate, Valentine works out Thomasina's diagram and reluctantly shares Thomasina's genius with Hannah.
- Gus is the self-elected mute son of Sidley Park. Gus is a connector and communicator between past and present; Gus intuitively knows where the foundation for the ruined outbuilding is. He dresses Augustus in Regency clothing and gives Hannah the trans-generational apple that Septimus eats. Gus is also present in the historical story—the actor is double cast as Augustus Coverly.
- Augustus, aged fifteen, is double cast with Gus Coverly. Augustus only appears in one scene as Thomasina's unruly young brother who wants Septimus to tell him about sex.