@ExeiL,

The official data book lists Alluka's gender as male, and two of Alluka's brothers, Illumi and Milluki, refer to Alluka as their brother.

However, Killua, the person closest to Alluka, specifically states that Alluka is a girl and refers to Alluka as his sister multiple times. While Alluka may have been designated male at birth, Killua's interactions show that Alluka may identify as female.

This inconsistency may be due to Killua's intimate and understanding bond with Alluka, hence he would know and care that Alluka identifies as female, versus Illumi and Milluki's cold and dehumanizing attitude toward Alluka.