exhibits dry humor—such as when he jests that Hannah is his fiancée or takes his pet turtle "Lightning" out for a "run"—Valentine is primarily a serious, analytical person. He is deeply inspired by the marvels of science and considers Bernard's quest to uncover Lord Byron's history as "trivial," arguing that it's not thepersonalities that matter, but the knowledge they contribute.

While Hannah seeks to uncover the reasons behind the downfall of Romanticism and its link to the Sidley Park hermit, and Bernard scrambles for evidence to support his wild theories about Lord Byron's flight from England, Valentine focuses on precise, calculated statistics—specifically, his family's game books. These books are a historical record, spanning centuries, of the animals hunted and killed on the estate. Valentine is analyzing this data to identify patterns in the life cycles of grouse in the region. He believes a formula describing these cycles must exist, which would bring order to chaos. Like Hannah, Valentine becomes engrossed in the research Thomasina conducted in the house two centuries earlier. Initially, he is skeptical that she understood what she was doing, as the scientific theories she proposed had not yet been discovered. "There's an order things happen in," he insists, "You can't open a door till there's a house." Ultimately, however, his scientific convictions are challenged, and he acknowledges the brilliance of Thomasina's ideas and their implications for the universe.

Lady Croom is the sharp-tongued and clever aristocrat residing at Sidley Park during the 1809 scenes. Despite her noble birth and refined upbringing, she frequently misquotes the artist Nicolas Poussin, offends her guests, and is willing to lower herself like any other character to fulfill her desires—primarily by engaging with any man who ventures into her dressing room. Her main aim in the play is to stop Richard Noakes from transforming the landscape around her estate into Lord Croom's idea of a Romantic wilderness. She prefers the current setup, featuring trees neatly clustered on the hillside and a meandering stream originating from an artificial lake in the middle of well-manicured meadows with an ideal number of sheep ‘‘tastefully arranged.’’ In her words, ‘‘It is nature as God intended.’’ Her perspective on nature reveals that, despite her sharp wit and eloquent conversation, she often unwittingly contradicts herself.

Lady Croom also seems to pursue casual romantic liaisons. Throughout the play, her name becomes linked with at least three of her guests: the poet Lord Byron, her daughter's tutor Septimus Hodge, and Count Zelinsky, a Polish aristocrat living abroad who is employed as Sidley Park's piano tuner. Septimus appears to take his relationship with Lady Croom seriously, as evidenced by a love letter he penned to be read in the event of his death before dueling with Chater and Captain Brice. However, like many before him, he is left behind when Lady Croom shifts her affections to Count Zelinsky by the play's conclusion.

Septimus Hodge, after studying mathematics and natural philosophy at Cambridge alongside Lord Byron, took up a position at Sidley Park as a tutor for Thomasina Coverly, the daughter of the Croom family. Septimus is portrayed as young, intelligent, witty, and apparently quite attractive. At the start of the play, he is twenty-two years old. A brief encounter with Mrs. Chater in the estate's gazebo becomes juicy gossip among the servants, and he is also engaged in an ongoing affair with Lady Croom, the mother of his pupil.

In the play, Septimus’s pursuit of knowledge is often rivaled by his indulgence in physical passions. His primary duty is to Thomasina, an exceptionally talented student, whose challenging questions and ideas frequently test his capabilities. While exploring mathematics and seeking proof for Fermat's last theorem, Thomasina curiously asks about the term ‘‘carnal embrace.’’ Septimus skillfully evades the awkward inquiry with a technically accurate, albeit misleading, response. "Carnal embrace is the practice of throwing one's arms around a side of beef," he humorously tells his inquisitive student. His quick thinking often helps him navigate tricky situations. When confronted by Ezra Chater, the husband of the woman he was caught with in the gazebo, Septimus admits his wrongdoing but cleverly manipulates Chater’s vanity. To avoid a duel over Mrs. Chater, whose honor, Septimus argues, "could not be adequately defended by a platoon of musketry deployed by rota," he proposes to write a flattering review of Mr. Chater's poetry collection, "The Couch of Eros," which Septimus actually despises.

Similar to Valentine in the modern segments, Septimus initially doubts Thomasina’s attempts to bring order to chaos in the universe through a simple mathematical theory. While he doesn’t question her creativity or intellect, he is more at ease when she adheres to conventional lessons from her books. Although he doesn't immediately recognize it in Thomasina, Septimus believes in the existence of genius. He views it as a quality shared by humans throughout history, with great ideas forming part of the ongoing continuum of life. "The missing plays of Sophocles will turn up piece by piece or be written again in another language," he assures Thomasina. "Ancient cures for diseases will reveal themselves once more. Mathematical discoveries glimpsed and lost to view will have their time again." Eventually, he realizes Thomasina is correct, and her theory implies the eventual demise of the universe. Yet, what he grieves is not the end of existence but the loss of innocence. "When we have found all the mysteries and lost all the meaning, we will be alone, on an empty shore," he laments before sharing Thomasina’s first, and final, waltz.

Hannah Jarvis is a composed and skilled historian, known for her seemingly unbreakable demeanor. She has been invited to Sidley Park by the present Lady Croom to investigate the estate's landscape transformations over the past 200 years. Her area of expertise is the study of landscape and literature from 1750 to 1834, and she has authored Caro, a best-selling book on Lord Byron. Despite not being an academic, her success as a field researcher and author has angered professors and aspiring literary critics across England. Now, she has stumbled upon a new subject. While exploring the Croom estate’s libraries and grounds, she has uncovered a mystery to delve into. She is on the hunt for clues about the Sidley hermit, referring to it as "my peg for the nervous breakdown of the Romantic Imagination."

Hannah's quest gains momentum when she teams up with an unexpected partner—Bernard Nightingale, an arrogant college professor who previously wrote a harsh critique of her book and is now seeking evidence of a Lord Byron scandal. Despite their contrasting personalities and frequent disagreements, Hannah and Bernard manage to assist each other in piecing together their respective mysteries. A major distinction between them, however, lies in their respect for the research process and historical documentation. Bernard is eager to publish his findings without possessing all the necessary details, whereas Hannah patiently seeks additional information to connect a small drawing of a hermit from one of Lady Croom's garden books to Septimus Hodge, an author, tutor, and, in Hannah's view, a representation of Romanticism's decline into the era of scientific reason.

In his review for the New Yorker, critic John Lahr described Bernard Nightingale as "a whirlwind of spurious intellectual connections" and "a literary climber of the first order." Other critics have labeled him as greedy, egotistical, and unpredictable. Bernard embodies all these traits and more. He holds a professorship at Sussex University, but his true passion lies in publishing, not teaching. When questioned about whether teaching should be a professor's main focus, he sarcastically replies, "Good God, no, let the brats sort it out for themselves."

Bernard serves as a satirical depiction of the worst type of scholar academia can produce. He is an irresponsible intellectual elitist who eagerly pieces together scant clues from the flimsiest evidence to craft grand theories aimed at gaining fame and inciting envy among his peers. To top it off, he dresses the part. Bernard arrives at Sidley Park in typical Sussex don attire—suit, tie, and a large leather satchel—enhanced by his own flamboyant touches, such as a peacock-colored handkerchief flamboyantly displayed from his jacket pocket.

He is at Sidley Park to gather evidence for his latest ambitious theory: a supposed connection between the renowned Romantic poet Lord Byron and a guest at Sidley Park in 1809. He enlists the assistance of Hannah Jarvis, a writer researching the estate's history, and the manor's current residents, descendants of Thomasina Coverly. During his stay, he also finds time to seduce eighteen-year-old Chloe Coverly. Together, they uncover a series of clues that might or might not support Bernard's theory that Byron killed a mediocre poet in a duel at Sidley Park in 1809, then fled the country for two years. Ignoring Hannah's caution that his evidence is insufficient for public disclosure, Bernard delivers a lecture to the Byron Society and even appears on a morning talk show. Shortly thereafter, Hannah discovers another clue that disproves his theory, shattering his hopes of achieving lasting academic fame—at least for now.

Captain Edward Brice

Captain Edward Brice is the audacious and boisterous brother of Lady Croom. Although he lacks his sister's sophistication and sharp wit, he shares her stubbornness. While on duty, he serves in the British Royal Navy, but during his leave, he resides at Sidley Park with his sister and pursues Mrs. Chater, the spouse of Ezra Chater. Due to Mr. Chater's even greater obliviousness, Captain Brice manages to carry on his affair with Mrs. Chater right under her husband's nose. At one point, when it seems Septimus Hodge might also be involved with Mrs. Chater, Captain Brice offers to defend Mr. Chater's honor in a duel. The twice-betrayed Chater remains unaware of the love triangle involving his wife. Eventually, when the Chaters are expelled from Sidley Park due to their scandalous conduct, Captain Brice offers Mr. Chater a position as a botanist on an expedition to the Indies. There, the unfortunate Mr. Chater dies from a monkey bite, allowing Captain Brice to finally marry his beloved.

Augustus Coverly

Augustus Coverly makes a brief appearance near the conclusion of the play. He is Thomasina's younger brother, fifteen years old in 1812, and attending Eton. When he first appears, he mocks his sister and is rude to Septimus. He returns briefly, however, remorseful and hoping the tutor will have a brotherly conversation with him about sex.

Chloe Coverly

Chloe Coverly is a member of the modern-day Croom family at Sidley Park. At eighteen, she is highly impressionable and immediately captivated by Bernard's flamboyant demeanor and assertive intelligence. Although she lacks the academic prowess of her older brother, Valentine, or the intuitive talent of her younger brother, Gus, she contributes one of the play's intriguing concepts. While everyone else seeks a coherent theory amid chaos, Chloe proposes that sex is the disruptive element. "The universe is deterministic all right, just like Newton said, I mean it's trying to be," Chloe asserts, "but the only thing going wrong is people fancying people who aren't supposed to be in that part of the plan." The human factor, as unpredictable as any chaotic force, is what others overlook. Ultimately, Chloe is swept up in the disorder when her mother discovers her in a "carnal embrace" with Bernard at the family garden party.

Gus Coverly

At fifteen, Gus is the youngest of the modern-day Coverly children, who are descendants of Thomasina and Augustus Coverly. He is autistic and non-verbal, typically shy but occasionally sociable. His brother Valentine tells Hannah that Gus spoke until he was five years old, then suddenly stopped. The current Lady Croom (who does not appear in the play) considers him a genius. After spending months and hiring specialists to locate the foundations of an old boathouse on her estate, Gus surprisingly led her directly to it. This mysterious boy seems to serve as a symbol in the play, possibly representing intuition over logic. Toward the end, it is Gus who gives Hannah the final clue she needs to solve her puzzle: a sketch of Septimus holding Plautus the tortoise.

Jellaby

Jellaby serves as the butler at Sidley Park during the 1809 scenes. He has few lines, and his primary role is delivering notes between Ezra Chater, Septimus, and Mrs. Chater. At one point, Septimus bribes Jellaby for information about the previous night's events, when Mrs. Chater was caught exiting Lord Byron's room, leading to everyone being escorted off the property.

Richard Noakes

Richard Noakes has a small role in the plot of Arcadia, with only a few lines, yet he embodies the Romantic spirit of his era. He is a landscape architect commissioned by Lord Croom to transform the grounds of Sidley Park from a tidy pastoral paradise, in the style of Capability Brown, into a wild, Gothic landscape, inspired by Salvator Rosa, a famed Romantic painter. Although the unseen Lord Croom appears to support Noakes and his vision for untamed, "natural" surroundings, the rest of the household is quite dismissive. Lady Croom frequently criticizes him, complaining about the noise from his new steam engine and mocking his design concepts, while Septimus calls him the Devil, saying, "In the scheme of the garden he is as the serpent."