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: 
Alluka is male. He just dresses like a girl.

From [Alluka](http://hunterxhunter.wikia.com/wiki/Alluka_Zoldyck#Gender_Ambiguity) article on Hunter x Hunter Wiki (emphasis mine):

> 
> There is conflicting information regarding Alluka's gender.
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> **The official data book lists Alluka's gender as male**, and two of Alluka's brothers, Illumi and Milluki, refer to Alluka as their brother.
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> However, Killua, the person closest to Alluka, specifically states that Alluka is a girl and refers to Alluka as his sister multiple times. While Alluka may have been designated male at birth, Killua's interactions show that Alluka may identify as female.
> 
> 
> This inconsistency may be due to Killua's intimate and understanding bond with Alluka, hence he would know and care that Alluka identifies as female, versus Illumi and Milluki's cold and dehumanizing attitude toward Alluka.
> 
> 
> 

In the episode showing Alluka in his childhood, we can see that he used to dress like a boy. He ended up looking like a girl, because he played with doll too much that his parent decided to dress him up like a girl.
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."