What Nationality Is Miss Brill
What is Miss Brill's nationality in "Miss Brill" by Katherine Mansfield?
There is no way to know for sure what nationality Miss Brill is. Katherine Mansfield does not include that detail in the text. What the reader does know is that the story is taking place in France and Miss Brill is there to teach "English pupils":
. . . and it also explained why she had quite a queer, shy feeling at telling her English pupils how she spent her Sunday afternoons.
Of course, the above quote doesn't necessarily mean that Miss Brill is teaching English. The line could be interpreted that she is teaching students who are English and living in France. She might be teaching history to English students. Or she might be teaching English (language) to French students.
We could assume that she is teaching the English language to French students, but it would then be possible for her to be English, American, Australian, or any other nationality that primarily speaks English.
I do tend to think that she is English, though. The usage of the word "queer" in the above quote is distinctly British in nature. My other reason is that the story singles out an "Englishman and his wife" at one point. Miss Brill likely has some experience and knowledge of what that nationality looks like in order to know, based solely on appearance, that the couple is English.
The character of Miss Brill in the short story of the same name Miss Brill, by Katherine Mansfield, is a single, middle-aged woman (we could categorize her as a "spinster") who dedicates herself to teach English in France.
However, we know from the story that Miss Brill is what is known as an expat (or expatriate) from England, which means that she is an Englishwoman who lives and works in another country.
To answer your question concisely, Miss Brill is a woman born in England and re-located in France where she works as an English teacher, perhaps as a teacher of English as a second language.