While Nanika can only grant one wish at a time, Nanika can fulfill multiple conditions for a single wish, greatly expanding the capacities of what one can ask of Nanika before granting three more wishes will. When a healing wish is expressed, Nanika must make physical contact with the target.
There are also additional rules for this Alluka/Nanika ability:
If Alluka makes requests from a specific person, it cannot be transferred halfway to another person . Therefore, Alluka cannot direct requests to someone else if that particular person disappears, for example by hiding.
Alluka has no Nen skills or Nen abilities of her own and only acts as a medium for Nanika to use her wish-granting powers. It’s unclear if these powers are aura-based and thus presumably Nen-related, or if they’re a separate type of ability.
Nanika has the ability to attach itself to and coexist in another living being’s body, taking control of them when certain conditions are met. Its physical abilities depend on those of the body it inhabits. It currently has the power of a normal child.
It has been confirmed that Nanika came from the dark continent and evidence suggests that Nanika is Ai, the desire for codependency. How Nanika started possessing Alluka we don’t know exactly, but seeing how Zigg Zoldyck went to the dark continent, it’s possible he brought it back to the outside world.
Alluka is the second youngest child of Silva and arguably the strongest member of the Zoldyck family. Although she doesn’t know how to fight, her frightening powers come from an entity that possessed her that Killua calls Nanika. Put simply, Nanika can do anything and everything.
Alluka wakes up when Nanika is sleeping. Nanika sleeps after a) before she starts making wishes after her wish is granted, b) when the person making her wishes also disappears from sight before hers can grant wishes or c) after Nanika has healed something and gets tired.
Kalluto himself was amazed that out of the entire troop, it was Feitan (who is a Transmuter, similar to Killua) who outclassed the youngest Zoldyck. Also interesting is that Kalluto is a weapon specialist like Feitan who uses his sword.