The two main characters in the novel are two brothers, Cosimo Piovasco di Rondò and Biaggio Piovasco di Rondò. The “baron in the trees” of the title is Cosimo, the elder brother; he is the protagonist. Biaggio, four years younger than Cosimo, is the narrator.

Three other members of the Piovasco di Rondò family are Arminio, their father; Corradino, their mother; and Battista, their sister. Outside the family, their neighbor Violate (or Viola) Ondariva plays an important role. This young woman is Cosimo’s great love.

The plot develops around Cosimo’s biography in the years he lives in the trees, beginning when he is twelve. The reader’s understanding of his character is controlled, however, by Biaggio’s position as narrator. The two brothers in some ways grow close, as Cosimo largely depends on Biaggio for food and information, yet both the practical realities of their lives and their worldviews increasingly diverge. The more Cosimo comes to symbolize rejection of the artifice of society, the more Biaggio epitomizes staunch endorsement of the status quo.

More extreme than Biaggio’s loyalism is Battista’s devotion to the artifice of upper-class society. She precipitates the crisis by preparing the dish that Cosimo deems inedible (snails) and other fashionable but unappealing concoctions. Her social-climbing aspirations are realized through her marriage to an aristocrat.

Their father, Arminio, is instead the embodiment of failed ambition, as he longs for the dukedom his family once commanded. In character, habits, and appearance, he harkens back to the glories of court life. Arminio does not merely reject but is utterly unable to comprehend Cosimo’s unorthodox antisocial life.

Their mother is Corradino, the so-called Generalessa, whose father was actually a general. While in some ways she is more stern and imposing than her husband, she manages to accept, if not actually understand, her son’s choices.