Hermes is a sentient motorrad, a type of motorcycle, and the constant traveling companion of the protagonist, Kino. The machine was originally found in a broken state in the Land of Adults by a traveler who would later sacrifice his life. This traveler, known as the first Kino, repaired the motorrad and named it Hermes after his previous vehicle. It was not until after its original owner was killed that Hermes first spoke, encouraging the young girl who would become the new Kino to escape the country on its back. In terms of its physical design, the motorrad is modeled after a real-world British motorcycle, the Brough Superior SS100.

Hermes possesses a distinct and often sardonic personality, frequently serving as a source of comic relief. It is characterized by a pessimistic realism and an unbiased outlook on the events it witnesses. A notable habit of Hermes is its tendency to misquote or incorrectly recall famous sayings and proverbs, which often prompts Kino to provide the correct version, suggesting the machine may make these errors intentionally. While it can be naive and take jokes literally, it is also highly knowledgeable, especially in matters of engineering and architecture, and has been shown to quickly perform complex ballistic calculations and assess the viability of foreign mechanical concepts, such as an airplane. As a machine, Hermes is not bound by human ethics or morality and has demonstrated an indifferent, pragmatic attitude toward suffering and death, once refusing to allow an extra passenger simply because there was no room.

The primary motivation for Hermes is to travel. It expresses happiness during maintenance activities like oil changes and cleaning, and has a deep appreciation for beautiful roads and buildings. Its fundamental purpose is to provide speed and transportation, requiring a rider to provide direction and balance. Hermes does not seem to care who its rider is, and if Kino were to die, it would simply wait for another traveler to come along. Kino, aware of this purely transactional role, fiercely protects both herself and Hermes from harm to ensure their journey can continue.

Hermes’ primary role in the story is as Kino’s sole, constant companion on an endless journey through various countries. It is the only being with whom Kino can share her innermost thoughts and have extended conversations, making it both a sounding board and a confidant. Their relationship is one of deep mutual reliance. While Hermes provides the physical means for travel and Kino provides the direction, their bond is also deeply psychological. Their dialogue drives much of the philosophical reflection within the narrative, as Hermes often questions humanity's customs and Kino's own reasons for perpetual travel. The key relationship is exclusively with Kino, though Hermes has briefly interacted with other characters, such as a dog named Riku.

Throughout the journey, Hermes demonstrates little to no emotional or moral development; its personality and worldview remain largely static. However, the bond of trust and understanding between Hermes and Kino deepens over time. Its development is more about the increasingly seamless partnership with Kino than any change in its own character. Its notable abilities include verbal communication, a deep reservoir of knowledge on mechanics and physics, the capacity to process numerical data with superhuman speed, and the ability to produce ultrasound beyond the range of human hearing, which it has used to chase away dogs. The true nature of its sentience, whether it be a spirit, advanced artificial intelligence, or a supernatural phenomenon, is never explicitly explained within the story.