Kathy, granddaughter of the infamous pirate Captain Flint, first meets Jim and Gran imprisoned in a slave pen on Pirate Island. Her capture followed dogged pursuit of her grandfather's treasure map. This heritage establishes her direct link to the central quest as Flint's heir seeking his legacy.

Feisty, resolute, and crafty, Kathy immediately reveals a strong will and deep distrust. Demonstrating agency, she swiftly seizes the treasure map from Jim after their escape, leveraging it to secure passage to the island despite lacking a ship. Throughout the voyage, she maintains guarded independence, resisting Captain Silver's persistent schemes to take the map. Her intelligence and resourcefulness flare as she repeatedly thwarts pirate plots, often aided covertly by Jim.

A critical shift occurs when Silver betrays and imprisons her. Under duress, she strategically negotiates cooperation, offering to guide Silver to the treasure's precise location in exchange for Jim and his friends' safety. This move signals a transition from self-reliance to protective loyalty. At the climax, she chooses to save Jim's life during his confrontation with Silver, sacrificing the treasure's final secret by triggering the mechanism that drains the volcanic crater lake. This act prioritizes human connection over material wealth.

Following the discovery of Flint's treasure within his sunken ship, Kathy departs the island alongside Jim aboard Silver's commandeered vessel. Her journey culminates in this partnership, marking a shift from solitary pursuit to shared purpose. Her character design and dynamic with Jim anticipate traits later associated with Hayao Miyazaki's heroines, emphasizing determination and moral complexity within adventure.