Nasubi is a young demon who serves as a prison guard in Japanese hell. In appearance, he looks like a small boy with silver-white hair, three short white horns on his forehead, large droopy eyes, and a cat-like mouth, but he is actually a fully grown adult. He belongs to a type of small demon known as a small oni. His speaking style is distinctive, as he frequently adds the syllable bi to the end of his sentences.

In terms of personality, Nasubi is cheerful, easygoing, and noticeably absent-minded. He has great difficulty focusing on work tasks, makes mistakes often, and is known for being an inveterate scatterbrain. Even after several years on the job, he has failed to learn the names and functions of the sixteen small hells, a fact that exasperates his more diligent colleague. His lack of focus and tendency to wander off into his own world make him a constant source of extra work for those around him. Despite these flaws, he is a genuinely good-natured and harmless character, and his cheerful, carefree attitude makes it difficult for others to stay angry at him for long.

Nasubi chose to become a guard primarily for financial stability, as his family came from a poor background. However, his most defining trait is a remarkable and unexpected talent for art. He possesses an almost genius-level ability in drawing, painting, and three-dimensional modeling. His artistic style is highly original and unfettered by convention, and he is particularly skilled at creating realistic and expressive works. He is an award-winning artist who admires the famous ukiyo-e master Katsushika Hokusai. This talent occasionally comes to the attention of his superiors, who recognize its value for special projects, such as renovating old murals or creating decorative objects for hell's facilities. He also learned a technique from the sage Hakutaku that allows his drawings to temporarily take on a semblance of life.

Nasubi's role in the story is that of a harmless troublemaker and a source of comic relief. His innocent but chaotic presence lightens the atmosphere and contrasts with the more serious or strict characters. His main relationship is with Karauri, his long-time friend and fellow guard. The two are often described as an inseparable pair, with Karauri acting as the responsible, hard-working one who constantly has to correct Nasubi's mistakes and guide him back to the task at hand. Despite the frustration, Karauri genuinely cares for his friend, and the two complement each other. Nasubi is also a subordinate of the demon Hozuki, the chief of staff. Hozuki, while usually strict and demanding, acknowledges and respects Nasubi's artistic gifts, sometimes trusting him with important creative assignments.

Nasubi does not undergo a dramatic character transformation over the course of the series, but his consistent portrayal reinforces the theme that hell, like any society, is a place where many different kinds of personalities can coexist and find a place. His absent-mindedness is balanced by his creative spark, and his continued employment shows that even an unconventional worker can contribute in his own way. His primary ability remains his artistic genius, which sets him apart from his peers and occasionally proves genuinely useful to the administration of hell.