Maggie is a ten-year-old fifth grader living in California who desperately wants a puppy so she can be the most important person in an animal's life. She struggles with intense emotions and a strong aversion to change, especially regarding her new school and the impending birth of her baby sibling. Upon discovering her severe allergy to animals with fur or feathers, she has to rethink her dreams and learn how to manage her condition while handling shifting family dynamics.

Claire is a sixth-grade girl who recently moved in next door to the Wilson family. She lives with her artist father in a quiet, organized house and only sees her mother during the summer and holidays due to her parents' divorce. She is friendly, outgoing, and enjoys the lively atmosphere of Maggie's home, offering a contrast to Maggie's own perspective on her family.

Sebastian is a science-loving fifth grader at Golden River Elementary who quickly befriends Maggie. Because he has a severe egg allergy that requires him to carry an epinephrine injection pen, he is highly knowledgeable about the immune system and medical facts. He approaches his medical condition with curiosity and acceptance, modeling a healthy way to handle chronic allergies.

Hannah is Maggie's mother and works as a social worker. Currently six months pregnant, she is frequently preoccupied with the impending arrival of the new baby, which inadvertently makes Maggie feel neglected. Despite these distractions, she is a dedicated mother who actively teaches her daughter emotional regulation techniques like deep breathing.

Ned is Maggie's father, who works as a computer tech. He is a caring and observant parent who manages a chaotic household with a sense of humor. When Maggie feels isolated or upset, he prioritizes listening and validating her feelings rather than immediately trying to fix her problems.

Liam is one of Maggie's younger twin brothers. He and his brother are a constant source of noise and chaos in the Wilson household, which often frustrates Maggie. Although he appears perfectly content just playing with his twin, he secretly desires a closer relationship with his older sister and feels hurt when she isolates herself.

Noah is the other half of the Wilson twins. Like Liam, he is boisterous and frequently intrudes on Maggie's quiet moments, such as accidentally hitting her with a pillow during family movie night. He is enthusiastic about the new baby and the possibility of getting a family pet.

Grandma is Hannah's mother and Maggie's grandmother. She is a frequent and helpful presence in the Wilson household, stepping in to watch the children when Ned and Hannah go to the hospital. She provides stability and comfort, offering medical drops to Maggie during an allergy flare-up and supervising the children's activities.

Dr. Idonjie is the allergist who diagnoses Maggie's severe animal dander allergies. She clearly and patiently explains the science of the immune system to Maggie, using a comic book to make the complex information accessible. She prescribes a long-term treatment plan that includes allergy shots to help mitigate Maggie's symptoms.

June is the newest addition to the Wilson family. During the early parts of the story, her impending birth is a major source of anxiety for Maggie, who fears the baby will take up all of her parents' time. Once she arrives, she becomes a central focus of the household and offers Maggie a chance to step into a nurturing role.

Pipsqueak is a pet mouse that Maggie sneaks into her bedroom to fulfill her desire for an animal companion. Because mice have fur, Pipsqueak triggers Maggie's allergies, forcing Maggie to shower constantly to hide her symptoms. She represents Maggie's determination to ignore scientific facts in pursuit of her dreams.