Louie is a ten-year-old boy who vows to save an orphaned newborn mini donkey from the brink of death. Despite a history of failing to keep small pets alive, he relies on relentless optimism to tackle the immense responsibility of feeding and medicating the animal. He deeply misses his older brother who is away in the military, frequently wearing his sibling's clothes and rereading postcards to feel a sense of connection.

Nora is a young girl who recently moved to town. She deliberately hides her appearance beneath a yellow ski cap and oversized, dirty clothes. Due to the tragic loss of her premature baby brother, she aggressively resists forming emotional attachments and bluntly insists that Louie's sickly donkey will not survive the winter.

Winslow is a newborn mini donkey whose mother died shortly after giving birth. He arrives at Louie's house weak, shivering, and unable to properly feed himself. As he slowly regains his strength, he bonds intensely with Louie and begins testing his environment with curious nibbling and increasingly loud vocalizations.

Gus is Louie's athletic and supportive older brother who has been serving in the army for the past year. Though he is physically absent from the household, his previous advice heavily influences Louie's daily choices. He communicates with his family through brief postcards that always end with the phrase, "Remember me."

Louie's mother tries to temper her son's wild optimism with practical reality. She reluctantly agrees to let Louie care for the newborn donkey in their basement but repeatedly warns him not to get his hopes up, fearful of the emotional devastation Louie will face if the fragile animal dies.

Louie's father is a pragmatic man who initially assumes the orphaned donkey will not survive the night. He allows his son to take on the responsibility of nursing the animal but often steps in as the voice of reason when neighborhood rules and practical constraints clash with Louie's hopes.

Mack is a thirteen-year-old boy who lives next door to Louie. Despite the three-year age difference, they share a steady friendship and look similar enough to be mistaken for brothers. Mack is increasingly distracted from their usual activities by his intense crush on a new girl in the neighborhood.

Claudine is Nora's elegant and pretty older sister who recently moved to town. She enjoys the novelty of the baby donkey and captures the attention of the neighborhood boys, occasionally finding Mack's constant presence overwhelming.

Uncle Pete is a practical farmer and a long-time friend of Louie's father. He owns the farm where Winslow was born and routinely deals with the harsh, unsentimental realities of raising livestock, losing animals, and managing predators.

Mrs. Tooley lives two houses down from Louie's family. She is frequently sleep-deprived and intensely protective of her baby, leading her to loudly and angrily complain whenever the growing donkey's braying disturbs the neighborhood peace.

Boom-Boom is Mrs. Tooley's infant baby. The child frequently cries, prompting loud defensive responses from the neighborhood donkey and frantic concern from his exhausted mother.

A local animal doctor who diagnoses Winslow with an infection early in the donkey's life. He is calm and pragmatic, taking the time to teach a terrified Louie how to properly administer daily medical injections.