Shenanigan is a mischievous young girl living at the centuries-old Swift House. True to the family tradition where dictionary names determine destiny, she fights against the expectation that she is inherently meant to be a troublemaker. She possesses a keen eye for observation and dedicates her time to hunting for the lost treasure of her ancestor, Vile Swift. When violence strikes the family reunion, she turns her investigative skills toward catching the culprit.

Candour is a handsome, cordial doctor who returns to the Swift family estate with his fiancée, Daisy DeMille. He presents himself as an ambitious but affable relative who makes puns and tends to his family members' physical health. He openly aspires to become the next patriarch of the family and seeks Aunt Schadenfreude's approval, entering into direct conflict with her when she refuses to bless his marriage.

Aunt Schadenfreude is the dour, strict matriarch of the Swift family. She wears dark, funereal clothing and a heavy iron collar that signifies the burdensome responsibility of her leadership. She meticulously manages the sprawling family's disputes and expects strict adherence to order, making her a frequent critic of Shenanigan's chaotic behavior. Her sudden announcement that she will choose a successor throws the reunion into an uproar.

Daisy is a beautiful American heiress engaged to Candour Swift. Despite her radiant, spring-like appearance, she harbors deep anxiety and guilt regarding her engagement and her standing within the eccentric Swift family. As an outsider unfamiliar with the family's bizarre traditions and deadly history, she struggles to endure the reunion and frequently relies on the kindness of relatives like Flora for support.

Phenomena is Shenanigan's older sister and the resident scientist of Swift House. She prefers logic, chemistry, and strict observation over the family's superstitions. When a crime occurs at the estate, she approaches the investigation with analytical rigor, though her confidence in her own intellect sometimes leads her to dismiss Shenanigan's contributions. She views her family through a clinical lens, meticulously tracking her younger sister's behavior in a notebook.

Felicity is the eldest of the three Swift sisters living at the estate. Because she possesses a relatively ordinary name, the family assumes she is destined for a dull life, leading to her being marginalized by older relatives. She loves fashion design and French magazines, using clothes to express herself. She frequently tries to act as a responsible guardian for her younger sisters, putting her at odds with the chaotic Shenanigan.

Fauna is a kindhearted and deeply compassionate member of the Swift family. As a transgender woman, she understands the difficulty of living up to rigid familial expectations and actively supports relatives who feel alienated by the Swifts' strict adherence to dictionary-ordained destinies. She frequently steps in to resolve conflicts with a calm dignity, prioritizing the emotional well-being of the family over traditional customs.

Flora is Fauna's identical twin sister. She is sharp-witted, fiercely protective of her twin, and highly suspicious of the extended Swift family's motives. Flora quickly recognizes the danger present at the reunion and advises the younger children to keep their heads down. She proves willing to intervene in family drama to protect vulnerable outsiders.

Uncle Maelstrom is a former sea captain who now resides at Swift House. He is a jolly, adventurous figure whose life path perfectly aligned with his dictionary name. He secretly encourages Shenanigan's curiosity by asking her to map the sprawling, secret-filled manor. Despite his cheerful exterior, he harbors deep anxieties about the family's financial state and the hidden dangers within the estate.

Aunt Inheritance is the strict family archivist tasked with preserving the Swifts' extensive history and maintaining their traditions. She firmly believes in nominative determinism—that a Swift's dictionary name dictates their entire life—and uses this belief to enforce order. She prefers handling family crises internally according to ancient rules rather than involving outside authorities.

Erf is a nonbinary child around Shenanigan's age attending their first family reunion. Rejecting the dictionary name they were assigned at birth, they chose "Erf" because it means a plot of land where anything can grow. They quickly form an investigative team with Shenanigan and her sisters, providing a grounded perspective while enduring their own anxieties about their strict grandmother's expectations.

Cook is a woman who ran away from Buckingham Palace in her youth and has lived at Swift House for two decades. Though she is not a Swift by blood or name, she acts as a caring parental figure to the girls living at the estate. The visiting relatives stubbornly treat her as a common employee rather than family, a classist distinction that causes tension during the reunion.

Gumshoe is a private detective who dresses in a stereotypical trench coat and fedora. He is hired by Aunt Inheritance to handle the family crisis internally. He relies more on hard-boiled detective tropes and dramatic declarations than actual investigative skills, inadvertently destroying crime scenes with his clumsy methods.

Pamplemousse is an eccentric French relative who attends the reunion dressed in a frock coat and armed with a sword, pistol, and daggers. He brings an extreme sense of honor and theatricality to the gathering, dramatically challenging the unknown traitor to a high-stakes duel of Scrabble.

Atrocious is an affluent, snide member of the Swift family. She openly acknowledges the dark, blood-soaked history behind the family's wealth, maintaining a cynically polite facade while insulting her relatives. Despite her abrasive personality, she possesses a practical understanding of the family's cutthroat dynamics.

Pique is the wealthy and arrogant brother of Atrocious. He is quick to complain about inconveniences, such as when he discovers the steering wheels have been stolen from the guests' cars, trapping everyone at the estate.