Leonard is a 300-year-old alien made of pure energy who mistakenly arrives on Earth trapped in the body of a striped cat. Instead of his planned trip to Yellowstone as a human park ranger, he ends up stranded during a storm in South Carolina. He possesses a poetic soul and a fascination with human culture, particularly rain and movies. He experiences physical pain and fear of mortality for the first time while observing human behavior from his new feline perspective.

Olive is an 11-year-old girl spending the summer with her grandmother in Hilton Head, South Carolina. She frequently wears overalls and daisy barrettes while caring deeply for animals. Critical comments from her mother's boyfriend have severely damaged her self-esteem, making her hesitant to connect with peers. She demonstrates remarkable empathy and bravery when saving creatures in need.

Norma is Olive's practical grandmother who manages the local aquarium in Hilton Head. A former shrimp boat captain, she maintains a tough exterior, wears vests with numerous pockets, and frequently calls people "sailor." She still grieves the loss of her son and works to bridge the emotional distance between herself and her granddaughter.

Frank is the new boyfriend of Olive's mother who works as a life coach. He openly criticizes Olive's interest in animals and characterizes her as socially unprepared for the real world. His harsh judgments serve as the primary source of Olive's insecurity and isolation.

Q works as an employee at the Hilton Head aquarium under Norma's direction. He dresses in bright Hawaiian shirts and approaches his work with upbeat enthusiasm. He actively encourages Olive, validating her interests and rejecting the idea that her personality is flawed.

Olive's mother usually resides in Maine but is currently spending the summer traveling. She keeps in touch with her daughter through letters and postcards. While she cares for Olive, she remains largely unaware of the emotional damage her boyfriend causes to her child.

Leonard's extraterrestrial species operates as a unified collective mind without individual emotion. They prioritize logic and data collection, granting members a one-month excursion to other planets for scientific observation. They reside in space among helium rivers and crystal mountains.

Stanley is Olive's large, fluffy dog. He establishes a silent understanding with Leonard early on, keeping Leonard's destructive mistakes a secret from the humans in the house.

Norma's late son and Olive's deceased father who passed away in a car accident. He shared a profound love for animals that he passed on to his daughter. Olive keeps a photograph of him in her pocket to feel close to his memory.

A friend of Olive's who lives near her usual home. She spends her time working with horses and goats, representing the quiet, animal-focused life Olive expected to have before relocating to Hilton Head for the summer.