- The book's heroine and protagonist, a homely, awkward, but loving high school student who is sent on an adventure through time and space with her brother and her friend Calvin to rescue her father from the evil force that is attempting to take over the universe. Meg's greatest faults are her anger, impatience, and lack of self-confidence, but she channels and overcomes them, ultimately emerging victorious.
Charles Wallace Murry
- Meg's extraordinarily intelligent five-year-old younger brother who is capable of reading minds and understanding other creatures in a way that none of the other Murry children can.
- A popular boy and talented athlete in Meg's high school who accompanies the Murry children on their adventure. Calvin comes from a large family that does not really care about him, but he nonetheless demonstrates a strong capacity for love and affection, and shows a burgeoning romantic interest in Meg.
- The disembodied brain that controls all the inhabitants of Camazotz with its revolting, pulsing rhythm. IT, identified with the Black Thing, is the embodiment of evil on this planet.
The Black Thing
- A cold and dark shadow that symbolizes the evil forces that Meg, Calvin, and Charles Wallace must fight against in order to rescue their father.
- The youngest of the three celestial beings who accompany the children on their adventure. Meg initially comes to know Mrs. Whatsit as the tramp who stole bed-sheets from their neighbors and then sought shelter from a storm in the Murrys' warm kitchen. She later learns that Mrs. Whatsit gave up her existence as a star in order to fight the Dark Thing.
- The oldest of the three celestial beings who accompany the children on their adventure. Mrs. Which has difficulty materializing and is usually just a shimmering gleam. Her unconventional speech is usually rendered in capitalized words, with the first consonants repeated several times.
- The second of the three celestial beings who accompany the children on their adventure. She usually speaks in quotations from famous thinkers and writers because she finds it too difficult to craft her own sentences. When the children first meet Mrs. Who, she is sewing sheets in the haunted house in their neighborhood.
- Meg's father and a physicist who works for a top-secret government agency on experiments with travel through space-time in the fifth dimension. In trying to tesser to Mars (i.e., travel through a tesseract, or wrinkle in time), he is captured and imprisoned on the dark planet of Camazotz. When the plot begins, no one on Earth has heard from him for over a year.
- This tall, furry, many-tentacled inhabitant of the planet Ixchel cares lovingly for Meg after she is nearly destroyed by the Black Thing. Aunt Beast, like all the creatures on Ixchel, lacks eyes and has no concept of light or vision.
- A jolly, clairvoyant woman in a silk turban and satin gown who shows the children a vision of Earth through her crystal ball. The Medium is reluctant to show them anything unpleasant, but the Mrs. W's insist that they see what they are up against.
Man with the Red Eyes
- A robot-like inhabitant of Camazotz who tries to hypnotize Meg, Charles Wallace, and Calvin in the CENTRAL Central Intelligence building. The Man, like all of Camazotz, is totally controlled by the power of IT.
- Meg's mother and an experimental biologist who works out of a lab in the Murry home. She is at once a brilliant scientist and a loving mother who cooks meals for her family on her Bunsen burner. She also writes loving letters to her absent husband every night.
- Meg's cold and unfeeling high-school principal who calls her "belligerent and uncooperative" and implies that her family is in denial about Mr. Murry's true whereabouts.
- The wife of the constable in Meg's hometown, who has twelve bed-sheets stolen from her at the beginning of the novel.
Sandy and Dennys Murry
- Meg's athletic and socially successful ten-year-old twin brothers who encourage her to let them fight off the bullies who make fun of Charles Wallace. The twins do not accompany Meg and Charles Wallace on their interplanetary adventure.