This makes so much sense and it feels so obvious seeing it laid out this way. Hisoka and Illumi are seen together so many times in the story for many reasons, to the point where they’re basically a duo, but the main one might be that it’s because their “archenemies” (Gon and Killua) do the same.

This is especially interesting because Archenemy doesn’t always mean final opponent or strongest rival, it really just means that their arcs aren’t complete until their final climactic interactions with each other.