I, Juan de Pareja is a novel rich in themes and populated with numerous characters, but its central themes revolve around Pareja. Trevino has mentioned that the book's central theme is that "the challenges of love across racial divides can lead to profound and loyal devotion." Throughout his life, Pareja seeks love, loyalty, and a sense of belonging—a place to call home. As a child, he offers his love to his first mistress, who dresses him as a doll and treats him like a cherished pet. Unaware of any other life, Pareja responds with loyalty. As he grows older, his emotional experiences become more complex.

During Pareja's journey from Seville to Madrid, where he will reside with his new master, Velazquez, he must earn his way by working for a muleteer. Under the harsh treatment of Carmelo the muleteer, he suffers greatly; he is starved, compelled to steal, and brutally beaten. He realizes that slaves are considered less valuable than mules, mere property without any rights. However, when Velazquez becomes Pareja's master, he treats him with compassion. Velazquez, a somewhat distant man absorbed in his art, is difficult to connect with; their relationship develops slowly over many years. This gradual development of their bond forms the core of the story.

Juan de Pareja is depicted as a sensitive and affectionate individual. Upon joining the Velazquez household, he befriends the family's two daughters, embracing them as if they were his own sisters. His master and mistress soon come to trust him. Despite his loneliness, he accepts his position as a slave and dedicates himself wholeheartedly to the well-being of the Velazquez family.

Pareja's compelling artistic drive sets this narrative apart from other stories about noble yet underappreciated servants who sacrifice themselves. From a young age, he embellishes his mistress's letters with illustrations. Inspired by Velazquez's brilliance, he dreams of becoming a painter himself. Sadly, Spanish law prohibits slaves from pursuing the arts; they may only be craftsmen, not artists. Pareja learns to skillfully prepare paints and canvases, and masters techniques for manipulating the light on a model and for setting up a painting's scene. However, his master insists he cannot be taught to paint what he sees. Driven by an overwhelming desire to create art, Pareja sells his most treasured possession—his mother's earring—to buy paints and brushes. He practices in secret, learning by observing his master. Eventually, Pareja can no longer bear the deceit and reveals his paintings to both Velazquez and King Philip. The evolving relationship between master and slave builds suspense, as Pareja risks severe punishment. As they grow to respect and care for each other, Velazquez acknowledges that Pareja has transcended his social position to become a cherished friend and remarkably skilled assistant. Velazquez then grants Pareja his freedom.

In the tale of Pareja and Velazquez, Trevino crafts a narrative of love that surpasses racial bias and the inherent brutality of slavery. She concludes that Pareja's extraordinary strength of character allows him to overcome the chains of slavery and emerge as a successful painter, with his works now displayed in major art museums. Pareja is a multifaceted character, so vividly portrayed that he seems to come alive. His intricate mind grapples with the significant issues of his era, transforming his personal experiences into universal themes and ideas that resonate across time. For example, Pareja longs for freedom, driven by the fear all slaves endure: "Will I be sold someday?" His anxieties about being unloved and abandoned mirror common human concerns.

Cruelty is a prominent theme in the novel. Pareja endures abuse due to his status as a slave...