Doniphan is a British student approximately thirteen years old, stranded on a deserted island with fourteen other boys after their schooner drifts from New Zealand into the Pacific Ocean. One of the group's older boys, alongside Briant and Gordon, he displays a competitive attitude that establishes him as a potential rival for leadership. This rivalry sometimes creates friction over decision-making and authority among the castaways.

He possesses practical skills and actively participates in exploration and survival efforts. Doniphan undertakes expeditions across the island, including a significant journey to the southern tip of Lake-Family. Accompanied by Cross, Webb, and Wilcox, he explores the Downs Badlands and follows the East-river, descending its left bank to reach the river mouth, highlighting his role in mapping the island's geography.

While resourceful, his pride and strong sense of independence contribute to interpersonal conflicts within the group, particularly with other natural leaders like Briant. His actions and choices form part of the group's challenges in maintaining unity while organizing essential tasks like securing food, building shelter, and preparing for winter.