Mr. Popper is a kindhearted, absentminded housepainter living in the small town of Stillwater. Though he has never traveled, he is deeply fascinated by the Arctic and Antarctic, spending his winter unemployment reading voraciously about Polar explorers. He balances his modest living circumstances with a boundless imagination and childlike enthusiasm, frequently distracted by thoughts of faraway places even while working. His correspondence with a famous explorer brings an unexpected addition to his household.

Mrs. Popper is a practical and pragmatic homemaker who works hard to manage the family's meager finances during her husband's seasonal unemployment. She attends meetings of the Ladies' Aid and Missionary Society and acts as the grounding force in the household. While initially stressed by the expenses and messes associated with a wild animal, she adapts by playing the piano to help train the birds and managing the logistics of their new life.

Captain Cook is a live, inquisitive Emperor penguin sent from the South Pole as a surprise gift. Named after a famous explorer, he possesses a mischievous energy and droll personality. He explores the Popper household by pecking at upholstery, eating the family goldfish, and building a rookery out of miscellaneous items inside the refrigerator.

Admiral Drake is a famous Antarctic explorer whose expeditions inspire Mr. Popper's daily daydreams. He serves as a distant catalyst for the story's events when he unexpectedly ships a live penguin to Stillwater in response to a complimentary fan letter. His adventurous, globetrotting life stands in direct contrast to the Poppers' quiet, small-town existence.

Janie and Bill are the young children of the Popper household. They are well-behaved and share their father's delight in the unexpected arrival of an Antarctic pet. The siblings eagerly assist in the animal's care, find entertainment in the bird's tobogganing antics, and eventually travel with their parents.

Mr. Greenbaum is the owner of a national chain of theaters who travels the country looking for new talent. He is a benign, opportunistic businessman who recognizes the entertainment value of the trained birds. He offers the family a lucrative touring contract that pays five thousand dollars a week to feature the performing penguins on his stages.

Greta is a female penguin procured from an aquarium curator when both she and Captain Cook suffer from the same mysterious, lonely malady. Her presence brings companionship to the household's original avian resident, leading to the rapid expansion of the penguin population in the Popper home.

Mr. Klein is a wealthy film company owner who seeks to feature the performing penguins in Hollywood movies. He promises a highly lucrative contract that could provide lifelong financial security for the Popper family, presenting Mr. Popper with a major decision about the birds' future.

Mrs. Callahan is a neighbor of the Popper family in Stillwater. She is initially frightened by the penguin during a neighborhood walk, mistaking him for an anteater or an exotic goose when he pecks at her striped stockings, but she later shows kindness when the bird falls ill.

Dr. Smith is the Curator of an enormous aquarium in Mammoth City. He recognizes that his own isolated penguin is failing rapidly due to loneliness and facilitates a solution that helps both his institution and the Poppers' beloved pet.