The unnamed narrator is an English-born boy of Norwegian descent who is orphaned at age seven following a fatal car accident. He moves into the care of his grandmother, developing a deep and trusting bond with her. He is observant and independent, spending his time building a treehouse or listening closely to his grandmother's warnings. He approaches the world with curiosity and accepts the terrifying reality of witches with practical caution rather than panic.

Grandmamma is a Norwegian woman and a retired witchophile who becomes her grandson's sole guardian after his parents die. She is an unconventional, cigar-smoking elderly woman who possesses extensive knowledge about the hidden society of demons posing as women. She prioritizes her grandson's safety above all else, sharing terrifying stories to educate him on how to survive.

The Grand High Witch is the supremely powerful ruler of the Secret Society of Witches. She demands unquestioning loyalty and orchestrates the global effort to eliminate children. Operating from the shadows, she travels the world disguised as a respectable, philanthropic lady while terrorizing both humans and her own subordinates.

The Witches are a secret society of demons who masquerade as ordinary women. They hate children intensely and use disguises such as wigs to hide their bald heads and gloves to cover their claws. They operate stealthily within human society.

Bruno is a young boy staying at the Hotel Magnificent with his affluent parents. He is highly focused on consuming food, frequently stuffing his pockets with chocolate bars and snacks. He openly boasts about his father's financial success and serves as a contrast to the disciplined narrator.

Mr. Jenkins is a wealthy, ill-tempered man vacationing at the seaside hotel with his family. He lacks imagination and frequently dismisses others, exhibiting a strong preoccupation with his own financial status.

Mrs. Jenkins is Bruno's mother, a woman who spends her vacation at the Hotel Magnificent. She is highly fearful of rodents and shares her husband's narrow, unimaginative worldview.

Timmy is the narrator's childhood best friend. The two boys collaborate on building a treehouse before the narrator's life is disrupted by the death of his parents and relocation to Norway.

William is one of two white mice given to the narrator to lift his spirits. The narrator trains him to perform tricks for a planned miniature circus.

Mary is the second white mouse gifted to the narrator. She participates in tightrope walking exercises behind a screen in the hotel ballroom.

Mr. Stringer manages the Hotel Magnificent in Bournemouth. He maintains strict rules regarding animals on the premises but capitulates when threatened with public health inspections.

Mrs. Spring is a woman hired to look after the narrator when his grandmother contracts a severe case of pneumonia. She provides temporary stability during a medical crisis.

Ranghild is a young girl from Grandmamma's past who disappeared after being approached by a tall woman wearing white gloves. Her story serves as a warning about the reality of witches.

Solveg is a child who accepts an apple from a stranger and subsequently becomes trapped inside an oil painting on her family's wall, aging inside the canvas before vanishing.

Birgit is a young girl who falls victim to a witch and transforms into a large chicken. Her parents are forced to keep her in a pen.

Harald is a child from Grandmamma's past who turns into a stone statue that his parents subsequently keep in their hallway as an umbrella stand.

Leif is a boy who turns into a porpoise during a family vacation, giving his siblings rides in the water before swimming away completely.