Alluka Zoldyck (アルカ, ゾルディック) is the second youngest sibling of Killua. He is said to be a dark, uncontrollable child with the ability to grant wishes but it can also cause large numbers of brutal deaths. Because of this vastly abnormal ability, he has been locked up in a vault, behind four 10-digit-keypad password-protected security doors at home. Killua wants to ask him to restore his best friend Gon, who is on the verge of death, back to normal, but his eldest brother Illumi currently wants to kill him. His younger brother Kalluto has apparently been trying to bring him back, perhaps not physically, but mentally, to a normal state so that he is no longer imprisoned by his own family.
Appearance
Alluka as a child
As of chapter 323, there is considerable confusion as to Alluka's gender. While he is mentioned by Illumi, Milluki and by the 2004 data book as a 'younger brother' and by the servants of the Zoldyck family as 'young master', Killua, the closest person to Alluka, refers to him as 'younger sister.' One possible theory to explain this is that he is male by birth and has dissociative identity disorder, and only Killua knows his female personality. It is stated by Killua in chapter 323 of the manga that the real Alluka calls Killua "onii-chan", whereas the one calls him "Killua" is something entirely different.
She/He has black eyes. In Killua's flashback of her/his childhood in chapter 322 and in the family photo of the Zoldyck children in chapter 229, Alluka had long, straight, unbound black hair and wore boy's clothing. However, at present she/he wears the traditional clothes of a Japanese shrine maiden (miko), boots and a headband with happy and sad faces on them; there are two small straight bunches of hair, each of which is fastened by 4 hair bands that have a smiling or sad face on them each, hung down on the sides of the front of his/her head.
Plot
Alluka's first appearance was in chapter 229, when a family photo of the Zoldyck siblings and their mother was shown to illustrate what Kalluto meant when he said he wanted to get his older brother back no matter how many years it would take. In the photo Alluka was not posing together with his/her mother and siblings, instead he/she was standing at a distance behind them and facing away. His name, however, was not shown in the manga until chapter 321, and for a long period of time readers of Hunter x 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.
The Zoldyck siblings and their mother
Alluka officially made his/her debut in the manga in chapter 321 in which he/she is seen sitting unemotionally on the floor of a room full of dolls. His/Her father, Silva, when talking to Killua about him/her, referred to her as an 'it' and said that he/she was darkness. Through Killua's flashback of his childhood in chapter 322, some background information about Alluka was revealed. As a little kid, Alluka once asked Mitsuba, a female attendant of the Zoldyck family, to do 3 simple things: carrying her, going up a staircase, and raising her up in the air. After Mitsuba did what she was told, Alluka's eyes and mouth suddenly turned pitch-black.
Alluka in wish-granting mode with pitch-black eyes and mouth
Freaked out by this, Mitsuba called for help and Killua came and changed her back to normal by asking her to make him go high-high, which she 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 when Alluka had 3 requests fulfilled, her eyes and mouth would turn pitch-black, and they would change back to normal only when she heard and granted a wish.
To test her ability, her 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 at the same time. Killua explained that this was the consequence of saying 'No' to her requests 4 times.
Some time afterwards, Yasuha, another female servant of the Zoldyck family, gave Alluka a piggyback ride at her request, thus activating her wish-granting mode. Giving in to her greed at the moment, Yasuha wished that Alluka would make her a billionaire. Alluka agreed and suddenly a large amount of banknotes started to rain down from a blimp in the sky. It was later reported that a currency transport vessal had suddenly gone missing and by the time it had been found all the money had disappeared, and the captain and crew kept giving nonsensical answers.
Getting interested in Alluka's power, Illumi asked for Silva's permission to deal with Kasuga, the next servant to take care of Alluka, in his own way. He told her to listen to Alluka's requests in exchange for his tolerance of her romantic relationship with an outsider to the Zoldyck family. When Kasuga was with Alluka, suddenly Alluka asked her for her liver, which Kasuga declined. Alluka followed it up with 3 continuous requests for her duodenum, spine and brain respectively, which were all declined too. As a result of Kasuga's refusal to fulfill those 4 requests, a total of 67 people, including herself, were killed simultaneously by Alluka's strange power.
Combat Abilities
Unlike the other children of the Zoldyck family who were trained from a very young age to possess such abilities as high resistance to poisons and electricity and great physical strength, it is stated by Illumi that Alluka has no combat abilities at all and he is just like a normal kid.
Nen Ability
Wish Granting
Alluka is a specialization user. He will grant one wish after three requests of his are fulfilled. According to Killua, the degree of wish that Alluka can grant is probably infinite. However, the greater the wish made to Alluka is, the greater his next three requests will become. The one that must bear the cost of the next three requests is not the one Alluka granted the wish to; it is the next person who must do it. And if that next person declines to fulfill Alluka's requests four times, depending on the difficulty level of the previous wish, at least two people, he/she and his/her best-loved one, will die.