Board Thread:Questions and Answers/@comment-25951143-20150101110046/@comment-17259909-20151229183644

Okay, I will give pieces of evidence for both sides here:

Alluka is a male:
 * Shounen Jump refers to Alluka as a Zoldyck brother here.


 * Mitsuba referred to Alluka as a young boy here.

Alluka is a female:
 * Killua explicity says "Alluka is a girl" here, and refers to Alluka as his little sister to most others.
 * Alluka uses atashi for self-reference, which is strictly feminine.