The narrator is a technical writer, former professor, and motorcycle aficionado. A few years before the story begins, the narrator suffers a protracted mental breakdown during which his personality undergoes a dramatic transformation. As the story progresses, the narrator becomes increasingly distressed about the possibility that his "former" (prebreakdown) self is returning to seize control of his consciousness. The narrator is a deeply philosophical thinker who uses the analogy of motorcycle maintenance as a metaphor for how to live the right way. He is also racked with guilt and anxiety about the damage his breakdown may have inflicted on his son Chris. He views the trip as a chance to sort his thoughts, develop his personal philosophy, and assess his role as a father.
Phaedrus is introduced in a few passing references that make it clear the narrator views him as dangerous. As the book progresses and the narrator recalls memories of his time as Phaedrus in greater depth, it becomes clear that until his breakdown, Phaedrus—whose childhood IQ was 170—is brilliant but so obsessed with defining Quality and the meaning of life that it causes severe mental illness. The narrator believes that after Phaedrus's stay in a mental hospital, his personality is completely transformed. Gradually, the narrator is able to process his past trauma and incorporate Phaedrus back into his consciousness.
Chris is a seriously troubled boy. He suffers from inexplicable stomach pains that may be a symptom of impending mental illness. He remembers a great deal of his father's breakdown but was too young to understand what was happening or why his father was so unapproachable. He is also too young for the motorcycle trip, feeling alternately bored and unsettled by his father's long silences. At one point, the narrator despairs of breaking through Chris's sullen unhappiness and decides to send the boy home. By the end of the book, the two have reconciled and regained hope about their relationship.
John is a musician who loves the romantic aspects of traveling by motorcycle but is unwilling to engage with the technical side of cycle maintenance. He is mildly irritated by the narrator's long discourses on philosophy but impressed at the same time by the narrator's eloquence. In some ways, John functions as a "straight man" for the narrator's long flights of fancy. He listens patiently as the narrator expounds on various ideas, but he never seems entirely convinced by what he hears.
Like her husband, Sylvia is romantic by nature and views machines and technology with suspicion. She tends to make overgeneralizations about the decline of modern society but seems more open to the narrator's ideas than does her husband.