Nattō Kozō is a small yōkai who belongs to the Nura Clan and lives in the Nura household. His most distinctive physical feature is his head, which is composed entirely of nattō, or fermented soybeans, and as a result he carries a strong smell of nattō wherever he goes. He stands about forty centimeters tall and weighs roughly four kilograms, making him one of the smallest members of the clan. He typically wears orange clothing that adds to his compact, childlike appearance.

In terms of background, Nattō Kozō has been a part of the Nura Clan for a very long time. According to certain accounts of the clan's early history, Nurarihyon once saved Nattō Kozō during a journey, and from that point onward Nattō Kozō began to follow Nurarihyon and call him captain. In some interpretations, this origin positions him as one of the very first subordinates to ever join the Nura group, making him a founding-level member in a symbolic sense. He has resided in the Nura house ever since, and his presence there stretches back decades.

His personality is gentle and loyal, though not particularly assertive. He does not seek leadership or combat; instead, he is content to stay close to the clan's leadership and offer quiet, dependable companionship. He shows no malice toward humans or other yōkai and seems to take pleasure in simple daily routines. His motivations are uncomplicated: he wants to remain part of the Nura Clan and serve its head in whatever small ways he can.

Nattō Kozō's role in the story is that of a background supporting character rather than a frontline fighter or major plot driver. He is most often seen accompanying Nurarihyon on outings, whether going shopping or visiting other yōkai figures such as Inugamigyōbu Danuki. He also served as a playmate for the young Rikuo Nura during Rikuo's childhood, indicating that he is trusted enough to be around the Nura family's children and is considered a harmless, friendly presence within the household. During the events of the Demon Capital arc, he appears in the Nura house and in various scenes where the clan prepares for or reflects on the conflict in Kyoto, but he does not participate directly in the major battles.

Key relationships define much of his character. His bond with Nurarihyon is the most significant; he is almost never far from the old yōkai leader and seems to regard him with deep loyalty and affection. His relationship with Rikuo is more informal and nostalgic, rooted in the days when he played with Rikuo as a small child. He interacts amiably with other household yōkai such as Kurotabō and Aotabō, but he does not have any particularly close or dramatic ties to them. He has no known rivalries or enmities within the clan.

As for development, Nattō Kozō remains largely static throughout the series. He does not undergo a personal arc or change his outlook significantly. His most notable early moment is his rescue by Nurarihyon, which established his lifelong loyalty. After that, he continues to fill his role as a steady, unobtrusive member of the household without growing in power, rank, or personality depth. His constancy is itself a trait that reflects the enduring, family-like atmosphere of the Nura Clan's inner circle.

Regarding notable abilities, Nattō Kozō does not display any combat-oriented fears, techniques, or supernatural powers that are demonstrated in the anime. His primary ability is his nattō head, which is edible according to some accounts, but he never uses it offensively or defensively. His smell, while strong, is more of a characteristic than a deliberate weapon. He has no recorded feats of strength, speed, or yōkai magic. His value to the clan lies in his loyalty and his long-established place within the group rather than in any martial or mystical capability.