It’s unclear why Kalluto joined the Troupe. It seems to have a relation to Killua : Kalluto wants to “bring his brother back”. The relation between his decision to become a Spider and how it will bring Killua back home are even more unclear.
But look at what Kalluto is thinking about when he think about bringing “his brother back” :
Killua is not the only sibling on the picture who had “disappeared”. Alluka is here, too.
What if Kalluto is looking for his “brother” because he doesn’t know that it’s actually his sister?
We don’t know how old Alluka was when her parents decided to lock her up and keep her outside the family’s business. On ths picture, we can deduce she’s around six or seven. She still wears traditionally masculine clothes.
It’s sadly obvious that, beside Killua, no one in the Zoldyck family is keen to use Alluka”s real pronouns. We can safely assume that, for Kalluto, at this age, Alluka is still his “brother” to him. If she had been locked up before she had been able to claim her gender to every family member, she remains a “boy” to him.
Now let’s take a look at the family chart by Killua :
It’s shown no relation between Kalluto and Killua. Not only it’s another argument for the theory - it’s not Killua that Kalluto is looking for - but also, if they weren’t that close, Killua probably never talked about Alluka’s gender to him.
And when I say “weren’t that close”, let’s remember that Killua didn’t even knew what nen was at an age when Kalluto already mastered it. Do this family even speak to each other at least once a year. Do kalluto and Killua even know how the other looks like. Seriously.
Now let’s talk about the really interesting thing : there is a relation between Kalluto and Alluka. A relation important enough for Killua to notice and write it down.
The chart is a good indicator of the family relationships, but also of Killua”s subjectivity. For example, as readers, we know that Milluki doesn’t obey his father - he didn’t kill when asked, he broke the rule by asking a computer to Alluka. Also, the relationship between Silva and Zeno is far more complex that just “give and takes.” Killua is just a child, lacks objectivity when it comes to his family - for good reasons - and “Secrets” could be every Zoldyck’s second name. Killua interprets the relationship as a one sided “envy”, but maybe, just like the other family’s relationships, it’s more complex than that.
Alluka and Kalluto are around one year appart in age. And between the non-gender conforming boy, or non-binary child, and the transgender girl, maybe a special bound has been formed.
In fanon, Kikyo encourages Kalluto gender expression. It seems logical : Kikyo has a very feminine fashion sense, and the mother and the child are very close. But what if this fashion taste actually came from Alluka? Kalluto sees his “bother” growing “his” hair, wearing dresses, cute hair ornements, and looks adorable as hell.
Because of course, eventually, she started to wear what she wanted as soon as she asked for it. Alluka is not the girl to whom you can say “no” easily, right? At least not four times in a row.
Alluka is so pretty, she has everything she wants, she doesn’t suffer the terrible trainings and is cherished by Killua. She has everything he wants. She is everything he wants - or so he thinks with his little child point of vue, little understanding of the situation. Children are very materailist sometimes. How Kalluto couldn’t be envious of her? And in the same time, how can’t he feel close to her?
And one day, suddenly, she disappears. She’s locked up, and no one has the right to see her without supervision. Alluka is too dangerous. She’s not family any more. Kalluto can’t be her brother anymore.
He misses her. Especially now that he’s allowed to wear kimono just as he always wanted. He wishes she could see him. But she can’t, not with this terrible power that lies inside of her, that can’t be separated from her. Kalluto accepts this fate, and get closer to his mother, the only other girl of his family.
Until one day, when his father returns from a mission against the Phantom Troupe, he talks about its leader who can extract his opponent’s power and keep it to himself.
Very little knows about the Ais by this time. Some knows about the parasitic nens. Maybe kalluto knows, and this is the closest of the nature of Nanika he can think about.
For Kalluto, this is the solution. He will join the Phantom Troupe, prove his worth, earn their trust, and will ask for a favor : break into the Zoldyck’s mansion, and get Chrollo stealing the “parasitic nen” from Alluka. Then, Alluka could “come back home” as his “brother”.
This is why he’s so diligent in helping to find an exorcist for Chrollo. This is why he’s so disappointed when he sees that he doesn’t have the level to impress the Troupe and be an authority. It will “take years” before he could ask the Troupe such a dangerous mission as opposing the Zoldycks - without hurting any, of course. But he’s ready to risk it all, without knowing it will lead to nothing.
Splurging on good bedsheets is so worth it. I spent more than I ever imagined I would last time I bought some, and they’re made out of eucalyptus fibers and I literally never imagined a bed could be like this. I can’t even describe what they feel like. I kind of regretted spending so much because I’m a fuckin cheapskate and spending money makes me nervous, but damn. So worth it.