User blog comment:Martialmaniac/Enough with Alluka!/@comment-10528819-20150220201831/@comment-5780397-20150221001334

I agree it is not confirmed, but there are more hints suggesting he is a boy.

Wait, Killua's mother is not blind, she merely wears a visor. She has been shown to know the posiion of people when they made no noise. As for the Alluka and Nanika matter, I think his split personality influenced him in regard to his perception of himself, but reading the chapters where he appeared again I noticed that Alluka is rather feminine even when not taken over by Nanika.