User blog comment:Olivemeister/Starting work on a page for Nanika/@comment-5780397-20170214134808/@comment-26871067-20170216234531

Especially not by Killua.

There is, admittedly, an argument that Illumi's single instance of calling Alluka a boy also refers to Nanika. At that point in the storyline, it's not known to either the audience or Illumi that Nanika is a separate being. In my opinion, the majority of the times where the Zoldycks talk about "Alluka", they are unknowingly referring solely to Nanika since they essentially revolve around "Alluka's" powers. They ignore who Alluka is as a person and focus entirely on her unnatural abilities, which we know to be Nanika. However, the line is about Alluka since Illumi doesn't seem to realize Nanika is a different being.

I don't think Togashi would put a line like that in and just trust his audience to figure out it Actually, Believe It Or Not, referred to Nanika when she wasn't mentioned by name. It is a kids' manga in the end so I think there has to be some meta "this probably wasn't a hint because it would fly over the target demographic's head" consideration.

So it's a weird situation in that Nanika is only gendered by someone who doesn't distinguish (or know to distinguish) between Nanika and Alluka. Other than that single line (where Illumi calls Alluka his younger brother), there's nothing even resembling gendering of Nanika. And there's definitely nothing gendered that targets Nanika specifically - it's all via association with Alluka.