The Medicine Seller, known in Japanese as Kusuriuri, is the nameless, wandering protagonist of the anime series Mononoke. He travels across feudal Japan, moving between the end of the Edo period and the Meiji era, carrying a large wooden box filled with medicines and a powerful Exorcism Sword. His true name is never revealed, and his origins remain mysterious, but visual clues such as his pointed ears, sharp canine teeth, and red facial markings imply that he is not entirely human. Some interpretations suggest he may be a kitsune or a being from the supernatural realm known as Shuuga, though nothing is definitively confirmed.

The Medicine Seller's personality is marked by a profound calm and detachment. He rarely shows strong emotion, even in the face of extreme danger or death, and maintains a stoic, almost clinical approach to his work. He is highly analytical and observant, preferring to work alone and relying on his deep knowledge of the occult to solve supernatural mysteries. Despite his cool exterior, he is not without compassion; he demonstrates empathy for those harmed by Mononoke and is willing to risk his own safety to protect innocent people. However, he also exhibits a wariness and occasional suspicion toward the humans he encounters, particularly when they are dishonest about their own roles in the spirits' manifestations.

His primary motivation is to seek out and destroy Mononoke, malevolent spirits that are born from the fusion of human souls with negative emotions. The Medicine Seller is driven by a need to understand the fundamental nature of existence, and he has expressed a deep existential fear that at the edge of the world, there may be no form, truth, or reason, which would mean he could no longer exist. This philosophical anxiety underpins his relentless pursuit of the three truths required to perform an exorcism: the Katachi, or physical shape of the spirit; the Makoto, or the truth behind its birth; and the Kotowari, or the reasoning and regret that keeps it bound to the human world. Only after uncovering all three can he draw his sword and destroy the Mononoke.

In the story, the Medicine Seller acts as a wandering exorcist and a catalyst for change. He does not simply banish spirits; he forces the humans involved to confront their own inner darkness and the societal injustices that gave rise to the Mononoke in the first place. His role is less that of a hero and more that of a detached investigator or mirror, reflecting the sins and traumas of those he encounters. The series is structured around his encounters with different Mononoke, each arc focusing on a specific spirit and the human tragedy behind it.

The Medicine Seller's key relationships are largely transient, as he is a solitary figure who rarely forms lasting bonds. The most notable exception is Kayo, a servant girl who appears in multiple arcs as a recurring human character. She serves as an audience surrogate, and the Medicine Seller seems to develop a degree of trust in her, relying on her for information and protection of other victims. His interactions with the Mononoke themselves are also significant, as he often shows a quiet respect for their tragic origins even as he prepares to destroy them. He also encounters a range of human characters, from desperate victims to corrupt officials, and his relationships with them are always professional and distant, though he sometimes acts as a catalyst for their personal reckoning.

The Medicine Seller's character development is subtle and largely static in terms of personal change. He does not undergo a traditional arc of growth or transformation. Instead, his development is reflected in the evolving nature of the Mononoke he faces, which move from simple vengeful spirits to complex manifestations of systemic societal corruption. This shift broadens the scope of his role from a simple exorcist to a commentator on human nature and social decay. Additional lore reveals that there may be multiple Medicine Sellers, with different individuals identified by the name of their sword, such as Ri or Gon, though this does not fundamentally alter the character's portrayal in the main narrative.

The Medicine Seller possesses a range of notable abilities. He is a master of occult knowledge and carries various medicines and tools from his box to fend off spirits. His most significant power, however, is the Exorcism Sword, which can only be drawn after he has successfully identified the Form, Truth, and Reason of a Mononoke. Once drawn, the sword transforms his appearance, revealing a more feral, demonic aspect that greatly increases his ferocity in battle. He also displays extraordinary abilities such as telekinesis, clairvoyance, and superhuman strength, despite his own claims of being ordinary. His overall power is such that he can perceive and navigate supernatural realms and always maintains an air of absolute control over even the most dangerous encounters.