Niko is one of the twin zashiki-warashi adopted by Hozuki, the chief of staff in the Japanese underworld. As a zashiki-warashi, a type of household spirit from Japanese folklore traditionally associated with bringing good fortune, Niko possesses a small, childlike appearance with distinctive features such as a short bobbed hairstyle and a simple kimono. Her personality is generally cheerful, playful, and somewhat innocent, yet she occasionally displays the mischievous and curious nature typical of her kind. She is closely bonded with her twin sister, Ichiko, and the two are often seen together, exploring the various realms of hell or interacting with the other inhabitants. Her adoption into Hozuki’s household places her under his strict but caring guardianship; he disciplines her when necessary but also indulges her and ensures her safety in the often chaotic environment of the underworld.

Niko’s role in the story is primarily that of a supporting character, appearing in the second season of the anime and related OVAs. Her presence adds a layer of domestic warmth and levity to Hozuki’s otherwise rigorous routine. She does not experience significant character development over the course of the series, but her interactions with Hozuki and Ichiko highlight her trusting and affectionate nature. Her motivations are simple: to enjoy her time in hell, play with her sister, and occasionally assist in minor tasks. As a zashiki-warashi, her notable abilities include the supernatural capacity to bring luck to a household, though this power is not explicitly demonstrated in the anime. She also appears to have the ability to move unseen or appear suddenly, a trait common to her species.