Alluka Zoldyck is the second youngest sibling of the Zoldyck Family. She 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, she has been locked up in a vault, behind four 10-digit-keypad password-protected security doors at home. Killua wants to ask her to restore his best friend Gon, who is on the verge of death, back to normal, but her eldest brother Illumi currently wants to kill her. Her younger brother Kalluto is envious of her, perhaps because of her closeness to Killua.
(Source: Hunter x Hunter Wikia)