Stig is the title character and deuteragonist, a prehistoric cave boy who lives in a den at the bottom of a modern chalk pit. He is a round but static character; while his personality and skills are multifaceted, his fundamental nature as a self-sufficient, ancient being remains constant. Because he does not speak a modern language, his character is revealed entirely through his actions, creations, and nonverbal interactions with Barney. Barney’s first impression of him as “not […] much bigger than himself, but he looked very strong, and his hands looked cleverer than his face” (11) highlights Stig’s embodiment of timeless human ingenuity and the ability to adapt. His existence in the present day is never explained in rational terms, positioning him as a figure of mystery and magic who challenges the boundaries between past and present. He is a powerful, almost elemental presence, whose skills and worldview offer a commentary on modern society’s relationship with waste and nature.

The most prominent of Stig’s traits is his extraordinary resourcefulness, which is central to the theme of The Transformative Power of Imagination and Resourcefulness. His den is a marvel of engineering, constructed entirely from the refuse of contemporary civilization. This ability to see potential in what others have thrown away is a form of practical magic. When Barney offers him a broken pocketknife, Stig’s instinct is to use the blunt steel blade to chip a piece of flint into a far superior cutting tool. This act symbolizes his connection to an ancient knowledge that transcends and repurposes the failures of modern technology. Stig’s world is one where an object’s value is created through skill and imagination, turning the dump from a symbol of waste into a treasure trove of possibilities.

Stig’s relationship with Barney is one of profound loyalty and unspoken understanding. He quickly recognizes Barney as a friend, and their bond deepens through their collaborative projects. Stig is fiercely protective, as demonstrated when he single-handedly terrifies the bullying Snargets and later chases off the thieves who burgle Barney’s grandmother’s house. While often depicted as a pragmatic survivor, Stig also possesses a rich inner life and an artistic capacity. He creates detailed and dynamic cave paintings of a hunt, losing himself in the memory and emotion of the scene. He also invents a primitive stringed instrument, suggesting an innate desire for music and expression. The novel’s climax during Midsummer Night reveals Stig’s origins, placing him within a tribe and a community engaged in a monumental ritual. This vision connects him to a larger prehistoric human history, showing that his skills and instincts are part of a deep cultural heritage.

Lou, Barney’s older sister, serves as a crucial foil to the protagonist and represents the encroaching rationality that separates the world of adults from that of children. As a dynamic character, her primary function in the narrative is to embody skepticism, initially dismissing Barney’s incredible stories with the logical, slightly condescending air of an older sibling. When Barney first describes his new friend, Lou’s immediate reaction is to re-categorize the experience into a familiar childhood game, concluding, “Stig’s just a pretend-friend, isn’t he, Barney?” (13). This response firmly establishes her position on the adult side of the theme The Divide Between Childhood Perception and Adult Skepticism. She is not unkind, but her worldview is governed by what is probable and known, making her unable to accept the existence of a cave boy in a dump.

Lou’s character arc is defined by her gradual and reluctant journey from disbelief to acceptance. While she initially resists Barney’s world, a series of extraordinary events forces her to confront the limits of her own understanding. The first significant challenge to her skepticism occurs during the hunt for the escaped circus leopard, where she, dressed in a leopard skin from Stig’s den, comes face-to-face with the real animal and witnesses Stig’s calm, capable presence. The pivotal moment of her transformation, however, is the shared adventure on Midsummer Night. Pulled into a time slip with Barney, she witnesses Stig’s tribe and their monumental task of erecting the standing stones. This direct, inexplicable experience shatters her rational framework and aligns her perception with her brother’s. By the end of the night, she is a co-conspirator, sharing a secret that she knows cannot be explained to adults. Her development from a skeptic to a trusted confidante completes her journey, suggesting that the capacity to believe in the extraordinary is not entirely lost with age, but requires an openness to experience.

Barney’s grandmother functions as the primary representative of the adult world in the novel. She is a flat, static character whose loving, gentle nature provides a safe, stable backdrop for Barney’s adventures. Her role is essential in illustrating the theme of The Divide Between Childhood Perception and Adult Skepticism. While she cares deeply for Barney, she cannot comprehend the reality of his friendship with Stig. She consistently interprets his accounts of Stig’s life and their shared projects as products of a vivid imagination. Her reaction to Barney’s first mention of Stig is one of amused affection, remarking, “What funny friends you have, dear!” (13). This response, while well-intentioned, places a firm boundary between Barney’s perceived fantasy and her adult reality. She humors him without ever truly listening, a common experience for children whose worlds are not yet governed by adult logic. Her consistent, benevolent disbelief highlights the solitude of Barney’s experience and makes his secret world with Stig all the more special and private.