Timothy is the 12-year-old eldest child of the Willoughby family. Attempting to compensate for his parents' total neglect, he acts as a harsh disciplinarian to his siblings, hiding a protective nature behind a bossy exterior. He creates arbitrary rules, such as deducting points for yawning, in a desperate bid to control the chaos of their household.

Jane is the six-year-old youngest child and only girl in the Willoughby family. Described as a timid girl with eyeglasses and bangs, she harbors a quiet determination to become more self-assured. She strongly dislikes being bossed around by her eldest brother and slowly learns to assert her own opinions, particularly when it comes to her food preferences.

Barnaby A is a 10-year-old twin who looks entirely identical to his brother. He shares a single, scratchy sweater with his twin and generally goes along with the ideas presented by his siblings. He survives his parents' apathy by sticking closely to his siblings and following their collective plans.

Barnaby B is the 10-year-old twin brother of Barnaby A. He possesses a sharp intelligence and frequently asks critical questions about their circumstances. Despite his parents' complete lack of effort to tell him apart from his twin, he maintains a thoughtful perspective on their old-fashioned predicament and the literature they read.

The Nanny is a stern, matter-of-fact, but ultimately fair caretaker hired by the Willoughbys. She brings logic, proper meals, and genuine interest into the children's chaotic lives. She is fiercely pragmatic and harbors a slightly dangerous hidden edge, showing an unconventional willingness to bend the rules to protect the children.

Mr. Willoughby is a bank employee and a thoroughly neglectful father. He detests his children and resents their presence, doing only the bare legal minimum to provide for them. Ill-tempered and selfish, he is constantly looking for ways to rid himself of his parental responsibilities, ultimately conspiring to abandon his family.

Mrs. Willoughby is a bitter and hostile mother who actively dislikes her offspring. She cooks terrible meals and possesses a completely unpredictable temper. She is self-centered and reckless, preferring to focus on dangerous vacation brochures from the "Reprehensible Travel Agency" rather than her family's well-being.

Mr. Melanoff is a wealthy, retired candy tycoon living in a state of profound squalor and depression. He considers himself an old-fashioned gentleman but has lost all interest in life due to his grief over his family, whom he believes perished in an avalanche. His chaotic, creative nature requires a purpose to thrive, which he finds when a baby is left on his doorstep.

Ms. Melanoff is a highly meticulous and particular woman living in Switzerland after surviving a terrible avalanche. She insists on strict alphabetical organization and total cleanliness. Her abrasive personality clashes with anything chaotic or messy, leading her to seek out a highly ordered life in her new environment.

Barnaby is a young boy displaced in Switzerland after a traumatic avalanche separated him from his old life. He struggles to adapt to his new environment, refusing to learn German properly and missing the chaotic, creative energy of his past. He is messy and adventurous, directly contrasting his mother's strict need for order.

Hans-Peter is a stuffy, meticulous postmaster who lives alone with his dog in a small Swiss village. He values order and symmetry, going so far as to ensure his sideburns are perfectly even. He harbors a romantic interest in the highly organized woman who visits his post office daily.

Ruth is a mysterious, abandoned baby who serves as a catalyst for change in the neighborhood. After Mrs. Willoughby demands the infant be removed from their property, the Willoughby children leave her at a gloomy nearby mansion. Her basic needs and cheerful disposition force the adults around her to step up and provide care.