Nyaa is a small black cat with a ribbon and a bell around its neck who is almost always seen in the company of the demon apprentice Shia. This feline appearance is a disguise, however, because Nyaa is actually a full demon whose true name is Klaus, a name that is attached to him in the broader series even though in the anime itself he is known only as Nyaa. The name Nyaa is given to him by the angel Misha, who calls him that because it echoes the Japanese sound a cat makes. He insists it is not his real name, but he tolerates the label throughout the anime.

Nyaa serves as Shia's mentor and supervisor. He travels with her to the human world to oversee her demon training and repeatedly urges her to behave in a properly wicked manner. Because Shia is gentle and benevolent by nature, she is unable to carry out any genuine evil, and her persistent kindness is a constant source of frustration for him. He is grumpy, easily annoyed, and quick to complain about the situations he is dragged into, yet underneath this irritation he is sincerely devoted to Shia and works to do what he believes is best for her rather than abandoning her.

When he appears in his human form, Nyaa is a young man with chin-length, messy blond hair and cold blue eyes, and his appearance carries a suggestion of German heritage. His past beyond that point is mostly unexplained, and he maintains a generally irritable disposition. He also holds a strong distrust or disdain for human beings, consistent with his demonic nature, and he reserves particular hostility for Sasha, the angel who supervises Misha. The mutual dislike between Nyaa and Sasha is open and ongoing, and their clashes are a recurring source of friction in the story.

Within the narrative, Nyaa is a supporting character whose presence is tied almost entirely to Shia. He does not drive the main events of the plot, but instead accompanies Shia, observes the chaos created by the angels and humans around them, and reacts to it with exasperation. His closest relationship is with Shia, to whom he acts as a protective companion and guide. His relationship with Misha is defined by her having given him his cat name, which he begrudgingly accepts, while his relationship with Sasha is defined by open antagonism.

Over the course of the anime, Nyaa remains a largely consistent figure. He does not undergo a major personal transformation, but his persistent efforts to teach Shia how to be a proper demon and his frustration at her inherent goodness highlight the contrast between the angelic and demonic sides of the story. His notable abilities include transforming between his cat form and his human form, and as a fully-fledged demon, he possesses powers and knowledge beyond those of a trainee such as Shia.