Hunter X Hunter is a story full of colorful characters. While its primary focus is on the adventure, it is also a story about relationships. They say opposites attract, which describes the pairing of Hisoka and Illumi well.
Hisoka is a flamboyant, bloodthirsty clown hailing from Meteor City. He grew up poor and little is known about his childhood other than his love for Bungee Gum. Illumi is the cold and reserved eldest child of the Zoldyck family. He grew up in a privileged home with a structure that was likely suffocating. Their personality traits and backgrounds set them up as great rivals, but their tendency to team up suggests compatibility. Perhaps these two just work better together.
The audience is first introduced to the duo during the "Hunter Exam" arc on Zevil Island. Gittarackur reveals himself to be Illumi, and Hisoka remarks that he enjoys watching the transformation. They seem to know each other pretty well at this point, and Illumi even gives Hisoka one of his extra badges. Then Illumi proceeds to bury himself, and Hisoka leaves to stalk more prey. They reconnect at multiple points throughout the series to help each other. The nature of their relationship remains elusive, much like the characters themselves. However, a closer look at their dialogue and understanding of how these characters think can help shed some light on the situation.
Illumi and Hisoka’s first meeting is likely never shown. The first clue given to the audience is after the Hunter Exam when Illumi comments that he has not known Hisoka very long. One could guess that they met during the exam, but it is hard to know. Illumi is never shown to have any associates outside of his family. For him to cozy up with Hisoka so quickly suggests an instant attraction or that they had met before. The second clue is during the "Yorknew" arc; Illumi mentions that he’s known Hisoka far longer than he’s known Chrollo. Because of this, Illumi chooses to help Hisoka over the Spiders, but for a price.
The third clue concerns a fan theory that Illumi changes his style after meeting Hisoka. Like Hisoka, Illumi wears a crop top, matching pants, and slide shoes. Considering that Illumi is often expressionless, this dramatic fashion better suits Hisoka. Flashbacks of Illumi show him looking very different, dressed in conservative attire with much shorter hair. Even the colors he wore were drab compared to the vivid green, red and purple he dons now. Next, there is a flashback of young Killua, who looks about six, with Illumi sporting dramatically longer hair and a green jacket. Perhaps everyone’s favorite clown had inspired the uptight Illumi to have fun and express himself a little more.
Fans of the pair should definitely read the "Succession" arc of the manga. At one point, Illumi joins the Spiders at Hisoka’s request. The spiders themselves sought out Illumi because they know he is close with Hisoka. Illumi tells Chrollo about their engagement and prenuptial agreement. This harks back to the "York New" arc when Hisoka jokes, "It’s not like we're getting married, just some give and take." Hisoka is speaking in terms Illumi understands since he views everything as a transaction. Illumi also describes his relation to Hisoka as "give and take."
Illumi likes to have total control of his situations. He uses business contracts, and when that doesn't work, he manipulates with his nen needles. Hisoka, however, is not concerned about structure, focusing instead on the thrill of the moment. He fixates on fighting, which he frequently associates with his libido. The prenuptial agreement promises a payout if Illumi kills him. This panel comes across as a cheeky, albeit strange, take on a traditional marriage, which is essentially a contract until death do they part. Illumi claims they set this up because he worried they would eventually kill each other. Considering that Hisoka is a whimsical liar who does as he pleases, it's unlikely to smooth sailing with the controlling Illumi.
While the relationship between these supporting characters is never fully addressed, Mangaka Togashi chose to use explicitly romantic terms. Therefore, it is perfectly valid for fans to interpret it as such, just as long as they remember who they’re dealing with. Hisoka and Illumi are both attractive and interesting, but they are also violent characters lacking a sense of morality. Fans can have their fun as long, as it doesn't translate into the real world.
