Shasta is the protagonist of The Horse and His Boy, though other characters are also well-developed. At the story's outset, Shasta is a young boy serving the only father figure he’s ever known, a fisherman. His fair hair distinguishes him in a community where most people have dark hair. A passing nobleman suspects that Shasta must have northern ancestry and cannot be the biological son of the fisherman he lives with. Much of The Horse and His Boy focuses on Shasta's journey to overcome his mindset of servitude, encouraging him to think for himself and act with autonomy. When Bree proposes they escape together, Shasta's motivation to leave is driven more by fear of the nobleman than a genuine desire for freedom.

In contrast, Bree yearns for freedom. He recalls being taken from Narnia as a young horse and forced to fight for Calormen in battles. Bree has shown immense self-control by remaining silent during his captivity, aware that revealing his ability to speak and think would attract unwanted attention from his master and others. However, he is somewhat conceited and has his own lessons to learn. While he has been a formidable warrior, his opponents were mindless animals, not the intelligent talking horses of Narnia, making them easy for him to defeat. Upon reaching Archenland, he realizes that being one of many talking horses will bring new challenges; he will find himself on equal footing with others, rather than superior.

Hwin is portrayed as a practical horse. Like Bree, she seizes the opportunity to escape Calormen. Bree reasons that a horse and a boy traveling together will draw less attention in Calormen than either alone, as people might be tempted to capture a stray horse or boy. Hwin shares a similar perspective regarding Aravis; she becomes a calming influence on the girl when Aravis considers suicide and serves as a stabilizing force for both Bree and Shasta as they navigate the emotional turmoil of transforming from slaves to self-driven, liberated individuals.

Aravis nearly upstages Shasta, the boy referenced in the title. She is fervent and headstrong, unlike other girls of her social rank, such as Lasaraleen, who finds the idea of being married off to a wealthy old man delightful. At some point, Aravis developed the notion that she should think independently and make her own decisions. While this is commendable, it comes with the harshness typical of Calormen nobility; she abandons a servant girl to be punished for allowing her escape. She also looks down on those of lower status, a sentiment she directs at Shasta. Like Shasta and Bree, Aravis has personal growth ahead of her. While Shasta learns self-reliance and Bree learns to coexist with equals, Aravis discovers that anyone with bravery and a kind heart deserves her respect and should be treated with courtesy.

Within "The Chronicles of Narnia," the portrayals of Susan, Lucy, and Edmund hold particular interest. High King Peter is absent, away battling the malevolent giants in the north of Narnia, and thus does not appear in The Horse and His Boy. Susan is momentarily captivated by the physical bravery of Rabadash, the Tisroc's son, who, during his visit to Cair Paravel, demonstrated his prowess as both a horseman and a warrior. Though Susan can occasionally be a bit frivolous, she is wise enough to recognize that the Rabadash she encounters in Tashbaan is a cruel individual who would mistreat her, as he would any woman. In Calormen society, women are scarcely more than men's possessions.

Notably, when Narnia dispatches troops to combat the Calormenes in Archenland, Susan remains at...