Question: In the anime *Hunter x Hunter*, Alluka Zoldyck is considered as a girl by Killua, shown when they leave their estate and were accompanied by four butlers. Killua shouted to Gotoh and said that he needs a female butler to join them because Alluka is a girl.

However, I am confused because Milluki address Alluka as a brother, in the episode when he wished to Nanika to kill the tourist photographer. He clearly told the the tourist that he was just taking his brothers for a walk. In addition, the [Hunter Wiki](http://hunterxhunter.wikia.com/wiki/Alluka_Zoldyck) acknowledged Alluka as a male.

What is Alluka's actual gender?

Answer: 
Actually, the *Hunter x Hunter* wiki [addresses this](http://hunterxhunter.wikia.com/wiki/Alluka_Zoldyck#Gender_confusion), and is quite accurate in its assessment, though the references cite the wrong volumes (but correct chapters).

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> There is considerable confusion about Alluka's gender. Two of Alluka's brothers, Illumi and Milluki, refer to Alluka as their brother. But Killua, the closest brother of Alluka, specifically states that Alluka is a "girl" and refers to Alluka as his sister multiple times. This inconsistency can be explained by Killua's intimate and understanding bond with Alluka, hence he would know and care that Alluka is mentally female, versus Illumi's cold and dehumanizing attitude towards Alluka. **While Alluka is most likely biologically male, Killua's interactions show that Alluka's psyche may be female.**
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Everyone (except Killua) addresses Alluka by her biological gender, which appears to be male. While she is quite small, she shows no female development.

However, she is very sensitive, gentle, and reserved, and has a very feminine side. Being that Killua is the only one who dares to actually interact with Alluka as a person, he would be the best one to understand that she probably is more feminine inside.

This does seem a bit odd to me; even if someone I knew were closer to "she" in his head, unless they specifically asked, I'd probably still say "he". But perhaps either Alluka did ask this of Killua, or Killua just took it upon himself because he knew Alluka so well.

Unfortunately, since it's never explicitly stated, this is somewhat assumption, but realistically it's the only explanation that makes sense since Killua is the only one to refer to Alluka as female.
Here is my assumption: When Alluka was born, her mother didn't take care of her, she let the butlers to take care of her (like bathing her, etc), and because their mother always thought that all her children must be male, so she thinks that Alluka is male too, especially because she is cold to her and never bathe her, so she doesn't know that Alluka is actually female, and the butlers are too afraid to tell their mother that Alluka is female. While Killua is the one who really cares for her, and knows her the most.

So even thought in the birth-biodata and everywhere, Alluka is stated as a male, but Killua knows the truth, and so the butlers who ever took Alluka to bath also have known that.

It's just my hypothesis after watching the anime. I think if this hypothesis is true, this story will be a very nice scene, because then it will be stated that "Even the writer of the story doesn't know about Alluka's gender, but her brother knows it because of how he always set an eye on her."