Dr. Hiriluk is a central figure in the backstory of Tony Tony Chopper from the Drum Island storyline. Before becoming a doctor, Hiriluk was a thief who was diagnosed with a fatal illness. Believing he was cured by the serene vision of a grove of cherry blossoms in a foreign land, he became convinced that no disease in the world is incurable. This experience led him to abandon his life of crime and dedicate himself to studying medicine, returning to his birthplace of Drum Island with the goal of healing the country.

In terms of personality, Hiriluk is famously eccentric. He is a self-proclaimed "quack doctor" who often tests his experimental concoctions on unsuspecting patients, which usually backfires spectacularly and forces him to flee. Despite his unorthodox and often harmful methods, his intentions are purely altruistic; he genuinely wishes to cure people and never asks for rewards. This compassion extends to animals, most notably when he discovers a wounded reindeer named Chopper. Even when the terrified creature attacks him, Hiriluk disrobes in the freezing snow to prove he carries no weapons, earning the reindeer's trust and taking on a paternal role. His most defining trait is his unwavering belief in his dream to cure the country's "frozen hearts," a sickness he believes was caused by the selfishness of the tyrant King Wapol.

Hiriluk's primary motivation is the completion of a lifelong research project. For thirty years, he worked to create a chemical powder that could turn the snow of Drum Island pink, creating the illusion of blooming cherry blossoms. He believed this miracle would heal the spirits of the kingdom's people, just as the real cherry blossoms had healed him. This dream becomes the driving force of his final days. He also acts as a surrogate father to Chopper, teaching him about medicine, the importance of a pirate flag as a symbol of conviction, and the core belief that a person does not die when they are forgotten.

Within the story, Hiriluk serves as the tragic inspiration for Chopper. He discovers Chopper when the reindeer is an outcast, hated by both humans and his herd, and gives him a home and a purpose. After a year of living together, Hiriluk learns that his own terminal illness has returned. To spare Chopper the pain of watching him die, he cruelly drives the reindeer away. In a heartbreaking turn of events, Chopper, trying to cure his father figure, returns with a poisonous mushroom that he mistakes for a universal remedy. Knowing the mushroom is lethal but moved by Chopper's love, Hiriluk proudly eats the soup made from it. With his death imminent, he goes to the castle after learning of a trap set by King Wapol. There, he delivers his famous speech on what it means to truly die, declaring that his dream will live on. He then drinks a volatile chemical, committing suicide by explosion so that Chopper would not feel responsible for his death.

Key relationships define his character. His bond with Tony Tony Chopper is the most significant, a deep father-son connection built on mutual love and stubbornness. He is a close friend and fellow free doctor to Dr. Kureha, whom he trusts enough to ask for medical advice and to whom he entrusts his research and the care of Chopper after his death. The royal guard Dalton initially views Hiriluk as a nuisance but comes to see him as a brave and noble man after witnessing his selfless sacrifice. King Wapol is his antithesis; Wapol views Hiriluk as a threat to his control and laughs at his death, representing the sick heart of the country that Hiriluk sought to cure.

In terms of development, Hiriluk transforms from a common thief into a man obsessed with a miracle cure, and finally into a selfless martyr for the sake of his country and adopted son. He begins his medical journey with wild, ineffective experiments but ends it by successfully completing the cherry blossom chemical, even if he does not live to see it used. His abilities are unique but unorthodox. He possesses considerable skill in chemistry, creating a range of substances from failed medicines to the successful cherry blossom powder and even liquid explosives. He is a poor physician in the traditional sense, lacking the surgical skills of Kureha, but his true strength lies in his ability to diagnose the metaphorical sickness of a nation. He is also proficient with firearms, specifically a tranquilizer gun and a rifle, and uses bombs to escape his many pursuers.