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A major theme throughout the Hunter x Hunter series is familial relationships. This can be seen as Gon spends the entirety of the series looking for his father, Ging. Kurapika hunting down the ones who destroyed his village and massacred the Kurta Clan follows this theme as well, and Leorio fulfilling his dream of becoming a doctor his childhood best friend could be proud of follows the same pattern. Despite being a bit different, Killua also follows this theme.
Although Killua hates his family and wishes to cut off all contact with them, besides Alluka, his older brother, Illumi Zoldyck, will not allow this to happen. Illumi continues to maintain a one-sided sibling relationship with Killua through lies, threats, manipulation, and many other mind-games that he enjoys playing. However, Illumi depicts a very strange and complex brotherly love for his younger brother, Killua.
Note: This article contains spoilers for the Hunter x Hunter (2011) anime.
Killua and Illumi have the most complex and toxic sibling relationship in Hunter x Hunter
Illumi is obssessed with Killua's actions
Since the beginning of the Hunter x Hunter series, Illumi has demonstrated a strange obsession with Killua. He constantly watches and follows him wherever he goes, on both the orders of their parents and his own accord. Illumi even went as far as to using his Nen abilities to change his appearance, creating the character Gittarackur. Illumi then underwent the 287th Hunter Exam, while teaming up with Hisoka Morow, to secretly watch Killua as he completed the incredibly difficult test.
Illumi is determined to make Killua the greatest assassin the Zoldyck family has ever seen, believing that with Killua's assassin training and his one-in-ten-million raw talent, Killua will be able to surpass not only him but Silva and Zeno Zoldyck as well.
As a result, he continuously analyzes Killua's actions to make sure he has not strayed from the path of becoming the greatest assassin the Zoldyck Family has ever known. However, Killua eventually loses the desire to become a great assassin, wanting nothing more than to travel the world with Alluka and become a good Hunter.
Illumi maintains control over Killua using manipulation and deception
Other than the Body Alteration ability he used to create Gittarackur, Illumi has another Nen ability known as Hypnotic Spell. This is how Illumi inserts a Nen needle into his target's brain, allowing him to control them using pre-made orders that condition his target into a certain way of thinking.
Illumi used this ability on Killua in an attempt to keep his younger brother on the path of becoming a powerful assassin. Illumi inserted a Nen needle into Killua's brain when he was a child. Ever since, when Killua would try and stand up against Illumi and his twisted beliefs to gain freedom from the Zoldyck Family, an overwhelming wave of fear would wash over him. This allowed Illumi to maintain a high level of strict control on all of Killua's thoughts and actions.
There are multiple moments throughout Hunter x Hunter where Killua is questioning the overwhelming fear he feels. Whenever he wants to fight an opponent in front of him to protect those he cares about, he would be forced to turn around and run.
Illumi convinced Killua that he would never survive in a fight against someone stronger than him. This was all part of Illumi's belief that assassins should immediately leave a fight if their opponent is deemed to be too dangerous in order to prioritize self-preservation.
However, during the Chimera Ant arc, Killua was able to resist Illumi's control during his fight with Rammot and eventually pull the needle out from his brain. This provided Killua with a moment of clarity. He was finally able to understand why he could never fight someone stronger than himself despite having the desire to. One could say that this was the moment where Killua finally broke free from everything Illumi used to hold him down.
Illumi has a protective nature for Killua
Despite the terrible things Illumi has done to his younger brother in Hunter x Hunter, he does not view any of it in a negative light and still cares deeply about him. In fact, Illumi justifies his actions of manipulating Killua as the only way to protect him. For instance, planting a Nen Needle in Killua's brain protected him from opponents much stronger than himself.
Illumi also believed he was protecting Killua from himself. By deciding to not be an assassin anymore, Illumi believed Killua was wasting his raw potential and ruining his life. As a result, he took matters into his own hands to try and force Killua back into the carefully calculated life Illumi and their parents had already planned out for Killua.
Final thoughts
Despite Illumi convincing himself that he loves Killua, he has put his little brother through some unimaginable experiences. Manipulating Killua for years using Hypnotic Spell and preventing him from ever making friends deteriorated their relationship immensely. Killua now hated Illumi more than anyone else in his immediate family and all of Hunter x Hunter, although their mother is a close second.
