Is alluka from hunter X hunter a boy or girl?

Alluka is most likely a boy though I have basically the same confusion as you. For starters Killua refers to Alluka always as a she and even uses the word “imouto” which literally translates to “little sister”. However every other member of the family refers to Alluka as a boy, even the butlers’ call Alluka “bocchama” which is basically a male specific “young master”.

Other than evidence directly from the manga there’s also the wikia which refers that Alluka's gender is male. Though its information is based on arbitrary criteria from the fanbase so I don’t take it as evidence. There’s also a character guidebook made by Togashi himself. I’ve seen pages but never the whole thing, neither do I know if the book comprehends every character that exists so far in the HxH world. If it did it’s not necessarily a fact that Togashi would include gender information but it´s a possibility and that would answer your question with a canon answer directly from the source.

So far I choose to accept Alluka as a sexually undefined and ambiguous character mainly because Alluka never talks about his/her gender nor does s/he ever responds in strange way to being called “little sister”, “little brother” or ”young master”. Though as I specified before it’s likely that Alluka is actually a boy.

Hunter X Hunter Alluka

I believe she is a girl and the confusion about her being a boy could be because of the English language translation. I believe..what I understand is that the rest of the family treat Alluka as something unhumen. Something that you refer to it as "it" like a table or an animal. Kailua in the other hand see her as a human and her strong power don't make her a freak of 'nature' like her family see. Which made them call her Something and not Alluka.

Am bored, someone reply to me haha

As long as Killua refers Alluka as a girl i'll stick with that.

Tsubone use "chama" to Alluka which means "cute female"

However, Illumi and Milluki, refer to Alluka as their brother.

Conclusion: Until the manga show me that there's no ****, i'll stick with Killua's description to her.

alluka is a girl.because she is sooooooo cute .for me she is also me to sister.so i refer her as a girl