- Victoria "Vicky" Austin is the protagonist and first person narrator, as she is in most but not all of the Austin family novels. Born in New York City, she lives with her family in rural Thornhill, somewhere in New England. She is twelve years old at the beginning of the novel. Her uncle Douglas describes Vicky as having an artistic temperament, but Vicky claims not to have a talent for anything.
- John Austin , Vicky's elder brother, is a budding scientist whose homemade space suit won a prize in a statewide Science fair. John (like Vicky) was born in New York City, when his father was 24 years old. John is fifteen years old as of chapter one of the novel. Vicky describes him as being "the nicest one of us all."
- Suzanne "Suzy" Austin , Vicky's sister, is just nine years old as the novel opens, but already knows that she wants to be a doctor. All of her games are oriented on her playing the doctor, including many mock operations on broken dolls. A year younger than Maggy, she is Maggy's chief friend and playmate. Suzy is blonde, and considered the beauty of the family. Suzy's refusal to eat pork after reading Charlotte's Web is based on an actual incident with L'Engle's elder daughter.
- Robert "Rob" Austin is Vicky's wise, loving baby brother, age four (almost five) as the story opens. The family enjoys his creative "God bless" litanies and other surprising things he says. His favorite toy is a plush elephant, named Elephant's Child after one of the Just-So Stories by Rudyard Kipling. The toy has a working music box, until Maggy breaks it. L'Engle acknowledged in her book A Circle of Quiet that the character is based on her son, Bion Franklin.
- Margaret "Maggy" Hamilton is the orphaned daughter of a jet-setting mother and a test pilot father. She is ten years old, loud and attention-seeking. She spends most of her time with Suzy, and the two often are punished for their mischievous ways. The character's background is one of privilege and neglect. Maggy has lots of toys but minimal discipline, and little contact with her parents. After Maggy's mother died of pneumonia during a trip to Spain, Maggy went to live with her father, Dick Hamilton, who died a month later.
Rounding out the family are Vicky's father, Dr. Wallace "Wally" Austin , a doctor in general practice; her mother Victoria Eaton Austin , a retired singer; an uncle, Douglas Austin , a painter who occasionally visits; Aunt Elena , actually a close family friend who is the Austins' connection with the orphaned Maggy; and Grandfather Eaton , a retired minister who lives in a converted stable on fictional Seven Bay Island.
