Nyamsas is a female cat who lives in the Kousaka household and functions as a supporting character in the story. The name also appears romanized as Nyamsus in some materials. She has a tuxedo coat pattern and wears a red bandana tied around her neck, and she is frequently described as a plump or overweight cat. Within the house, she enjoys a privileged position, since Junpei's mother and younger sister dote on her and spoil her thoroughly, a situation that stands in sharp contrast to the strained relationship she has with Junpei, the one family member who is allergic to cats and openly dislikes them.

At the beginning of the series, Nyamsas comes across as ill-tempered and seems intent on making life difficult for Junpei, often acting as an irritant in his everyday routine. Her behavior, however, is more complicated than simple hostility. After Junpei becomes able to understand cats as a result of breaking the statue of the local cat deity, Nyamsas starts to function as a guide for him. She is among the first cats to give him a request, joining the temple cat Tama in asking him to help the stray cats living in the local park who have been scared and scattered by a well-meaning but overwhelming cat lover. From that point onward, she becomes one of the main forces keeping Junpei connected to his task of fulfilling one hundred wishes from cats in order to lift his curse.

Her motivations are centered on the welfare of other cats rather than on cruelty toward Junpei. Her harsh and bossy manner is a form of gruffness, and as the series progresses, it is revealed that her true feelings are far more caring than her outward attitude suggests. Her role in the story is therefore that of a reluctant helper and local informant: she knows what is happening among the neighborhood cats, she relays their troubles to Junpei, and she effectively keeps him focused on his obligation while maintaining her prickly personality.

Nyamsas has several important relationships. With Junpei, she moves from apparent antagonism toward a bond built on mutual dependence, since he requires her guidance to fulfill his curse-related duties and she relies on him to act on behalf of cats in need. With Tama, she forms a steady partnership, frequently appearing alongside him when directing Junpei toward new requests. With Junpei's mother and sister, she is the pampered family pet who enjoys comfort, food, and constant attention, all of which the household readily provides.

Her notable abilities are tied to her knowledge of the local cat community and to her position as a communicator following the start of the curse. Like all cats in the series, she is able to speak with Junpei once he gains his ability, and she uses this to share information, organize help, and participate in the request system that drives the plot. She does not possess supernatural powers of her own; her significance comes from her place in the Kousaka home, her direct connection to Junpei, and her familiarity with the other cats in the area.

In terms of development, Nyamsas is a character whose exterior is gradually peeled back rather than fundamentally changed. The story reveals that her rough treatment of Junpei masks genuine concern for him and for other cats, and this reframing softens her comic antagonism without erasing it. She remains a strong-willed cat with a distinctive personality throughout, but the audience comes to understand the reasons behind her behavior and the loyalty that lies beneath it.