Jack Robinson is the youngest son in the anime's Robinson family. He is three years old when the family embarks for Australia, much younger than his ten-year-old novel counterpart. As the baby of the family, Jack primarily helps his mother, Anna Robinson, with daily chores and household duties on the island. He displays typical childhood curiosity and energy, often engaging in playful antics.

Jack forms a close bond with the family's adopted animals, particularly a baby cuscus named Mercer, who becomes his cherished companion. His youth often positions him as an observer during family endeavors, though he joins explorations and activities suited to his age, like accompanying his siblings on outings. When the volcanic threat forces a hurried departure, Jack relies entirely on his parents and older siblings for safety and decisions, highlighting his limited agency in survival scenarios.

Throughout the narrative, Jack's character does not experience significant individual development or take on major responsibilities like his older siblings, Franz and Flone. His role remains consistently that of the youngest member, contributing to the group dynamic through his presence and interactions rather than pivotal actions or leadership. The family's rescue and relocation to Australia conclude his island journey without specific details on his personal growth or future.