Jane is the youngest sibling and only girl in the Willoughby family at six years old, depicted as timid and bespectacled. She seeks to assert her independence and often clashes with her brothers, especially Timothy.

Barnaby A and Barnaby B are the 10-year-old Willoughby twins, known for looking identical and being referred to simply as "A" and "B." They demonstrate intelligence and adaptability, evident in their questioning nature and decision to change their names.

The nanny is a caring and logical figure who becomes a positive influence on the Willoughby children, guiding them and providing the structure that their parents fail to offer. She is characterized by her straightforwardness and resourcefulness.

Mr. Melanoff is a creative, somewhat chaotic man who is revitalized by the newfound presence of family in his life. He undergoes significant personal growth throughout the story, reconnecting with his passion for candy-making.

Mr. and Mrs. Willoughby are neglectful and selfish parents who show little interest in their children's welfare. Their disregard shapes the children's experience, as the siblings fend for themselves in their parents' absence.

Mr. Melanoff's son, Barnaby, is a resourceful young boy who navigates his way back to his father after years of separation. His adventurous spirit aligns with his father's personality, contrasting with his mother’s meticulous nature.

Ms. Melanoff, Mr. Melanoff's ex-wife, is characterized by her meticulous habits and organizational tendencies. Her relationship with her son becomes strained as he reminds her of her former husband.

Hans-Peter is a postmaster in Switzerland who forms a relationship with Ms. Melanoff, sharing similar meticulous traits. His presence in her life influences her decisions regarding her son.