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 she 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. Her younger brother Kalluto is envious of her, perhaps because of her closeness to Killua.

(Source: Hunter x Hunter Wikia)