Alluka Zoldyck

Alluka Zoldyck is the second youngest child of Silva and Kikyo Zoldyck. Alluka is a young child, with long black hair, blue eyes, and pale complexion. Alluka wears the traditional clothes of a Japanese shrine maiden, with boots and a headband adorned with cartoon faces; there are two small straight bunches of hair, each of which is fastened by four hair bands that each are decorated with the same cartoon faces on Alluka's headband, hung down on the sides of the front of his head. During Alluka's childhood, Alluka had long, straight, unbound hair, and wore boy's clothing.

There is conflicting information regarding Alluka's gender. 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.  Before Alluka was formally introduced in the manga, Silva refers to his child as a dark and uncontrollable child, that Alluka has no soul because of the circumstances of Alluka's birth, and cannot feel emotions. However, once reunited with Alluka's closest brother Killua, Alluka acts like a child with loving and lovable personality. Alluka displays a rather simplistic and childlike personality, contrasting with Killua's mature composure. Alluka often acts very happy with Killua, likely because he was the only one to show any compassion towards Alluka, and trusts him deeply. This implies Alluka has a compassionate personality, contrasting with Alluka's potentially horrific ability. According to Killua, Alluka calls him "Big brother", while Nanika calls him only by name.

Alluka's first appearance is in a family photo of the Zoldyck siblings and their mother. It was first shown in an earlier Chimera Ant arc chapter illustrating Kalluto's desire to get his older brother back no matter how many years it will take. In the photo Alluka was not posing together with the rest of the family, instead Alluka was standing at a distance behind them and facing away. Alluka's name was not revealed in the manga until chapter 321, and for a long period of time readers of Hunter × Hunter could only guess the name of the facing-away child based on the information about the Zoldyck family provided in the 2004 official data book. Alluka officially makes a debut appearance in the manga in chapter 321 in which Alluka is seen sitting unemotionally on the floor of a room full of dolls. Silva, when talking to Killua about Alluka , refers to him as an 'it' and says Alluka is darkness. Through Killua's flashback of his childhood, some background information about Alluka is revealed. As a little kid, Alluka once asked Mitsuba, an attendant of the Zoldyck family, to do three simple things: carrying Alluka, going up a staircase, and raising Alluka up in the air. After Mitsuba did what she was told, Alluka's eyes and mouth suddenly turned pitch-black. Freaked out by this, Mitsuba called for help. Killua came and changed Alluka back to normal by asking him to make himself (Killua) go high-high, which Alluka did. After the accident, Killua had to tell his parents about Alluka's secret because Mitsuba had already reported it to them. It turned out that when Alluka had three requests fulfilled, Alluka's eyes and mouth would turn pitch-black, and they would change back to normal only when Alluka heard and granted a wish. To test the ability, Alluka's mother Kikyo ordered Mitsuba to decline any request of her, no matter how small it was. Due to that, Mitsuba and her lover Hasama, who was not around them at that time, were mysteriously crushed into minced meat simultaneously. Killua explained that this was the consequence of saying 'No' to Alluka's requests 4 times.