Before the events of the recent manga arc, Hisoka and Illumi were associates, and the closest thing each other had to friends. They would often assist each other when the other needed, some instances of this being during the Yorknew City arc when Illumi disguised himself as Hisoka so that the latter could have a chance at fighting Chrollo, and during the 13th Hunter Chairman Election arc when Hisoka assisted Illumi's plans to chase down his brother by disposing of a number of Zoldyck butlers on his behalf. However, despite the mutual, "give-and-take" benefit of their relationship, Illumi always believed that one of them would wind up killing the other, and therefore, when Hisoka's motives changed, decided to form a contract with him with the condition that if the magician was killed, Illumi would receive his reward. Illumi refers to the job as an "engagement ring" and "prenuptial agreement."
It can also be of note, coincidentally or otherwise, that the two often wear the same or similar-shaped clothes, their preferred style being pants loose around the hips that match the color of a domed crop top, with a different-colored shirt worn underneath. In the Hunter Exam arc Exam arc there is also the addition of short, puffed sleeves to both of their outfits, and it is also likely that, during this arc, Illumi wore his hair slicked back to further match his appearance with Hisoka's. Also, during the Yorknew City arc, Hisoka wore Illumi's usual color, dark green, the one time Illumi wore a different-colored outfit for an arc.
It is important to note that Hisoka and Illumi are revealed to be engaged in the manga. This is proven by Illumi casually admitting to Chrollo that he and Hisoka's ordeal is an engagement ring.
The ship mostly formed due to these two characters' matching psychopathic natures, and the fact that in the majority of Illumi's appearances thus far, Hisoka has also been present, showing that the two apparently spend quite a bit of time together. It's difficult to tell if their interactions could at all be interpreted as vaguely romantic in canon due to Illumi's mostly monotonous appearance and Hisoka's blatant insanity, but it is evident that they have a reasonable deal of respect for each other and each other's abilities as a fighter, a major thing in the world of Hunter x Hunter. Perhaps the only time Hisoka has ever looked somewhat uncomfortable because of another person during the series' entire run was when he was discussing Alluka and Killua with Illumi, and the latter revealed that he thought he'd die if Killua made a wish rather than Killua's closest friend, Gon, after which Hisoka frowned and told him "I have issues, but you're just as bad."
As the most popular ship for both characters in the fandom by a large margin, HisoIllu is often chosen above other ships when it comes to the creation of fanfiction and fanart. It is largely shipped by anime-only fans, partly due to most fans of the series not having read the manga, but also because of the off-screen ending of the relationship between Illumi and Hisoka that took place in manga-only chapters.
HisoIllu is the first most written ship on AO3 for both Hisoka and Illumi.
|SHIPS||het||HisoMachi • Killunary • KuraSen • KuroNeo • OuMugi • Ponzukkle|
|Chrollinks • HisoGon • HisoIllu • Killugon • Leopika • PariGing • UvoShal|
|femslash||Amanary • Pakumachi|
|family||Hisoka x Abaki|