Marvin is a young beetle living behind the kitchen sink in a New York City apartment. He possesses a unique talent for drawing and an unusual curiosity about the human world. Unlike his extended family members who view humans strictly as hazards, Marvin is observant and empathetic, particularly toward the young boy whose bedroom he frequents.

James is a sensitive, observant eleven-year-old boy adjusting to life after his parents' divorce. Living with his mother, stepfather, and baby half-brother, he often feels overshadowed by his mother's social ambitions. When he receives a pen and ink set for his disappointing birthday, he unexpectedly forms a bond with a beetle.

Christina is a curator for the Drawings and Prints Department at the Metropolitan Museum of Art. Characterized by her tidy appearance and rectangular glasses, she holds a scholarly passion for the works of Albrecht Dürer. She enlists James's help for a specialized art project involving a staged theft.

Karl is James's father and a professional painter who lives separately from James following his divorce from Mrs. Pompaday. Though he sometimes struggles to connect with James's actual interests, he cares deeply for his son and is eager to encourage what he believes is James's budding artistic talent.

Denny is a rumpled-looking older man who works as the Curator of Drawings at the Getty Museum in California. He is visiting New York to assist with a special exhibition featuring works borrowed from his home museum. He shares a profound passion for Albrecht Dürer's art and assists Christina with her museum duties.

Mrs. Pompaday is James's mother and a status-conscious real estate agent. She prioritizes appearances and networking over her son's emotional needs, explicitly using James's birthday party to entertain her clients' children. She is eager to capitalize financially and socially on James's apparent artistic talents.

Mama is Marvin's caring and highly protective mother. She lives in the cupboard behind the kitchen sink and constantly worries about the dangers of the human apartment. She deeply values her family's safety and enforces strict rules to keep Marvin hidden from the human occupants.

Papa is Marvin's father. Like Mama, he is extremely cautious about the humans in the apartment and prioritizes his family's survival over exploration. He is proud of Marvin's unique abilities but shares his wife's anxiety whenever Marvin ventures out.

Mr. Pompaday is Mrs. Pompaday's second husband and James's stepfather. He is practical and sometimes skeptical, showing little appreciation for the modern art painted by Karl that hangs in the family's living room.

Elaine is Marvin's cousin who lives in the bathroom cupboard. She frequently plays with Marvin during family outings to the solarium, though her lack of swimming skills occasionally puts her in precarious situations.

Uncle Albert is Marvin's uncle and a skilled electrician within the beetle community. He helps guide Marvin safely to the bathroom during the dangerous mission to retrieve the lost contact lens.

William is James's baby half-brother. From the beetles' perspective, he is an excellent source of dropped food crumbs, making the floor around his highchair a popular foraging spot.