User blog comment:Stuffee/Illumi or Hisoka/@comment-24545282-20140214225545

Hisoka seems more mysterious to me because we know less about him than we do about Illumi. We've been introduced to the Zoldyck family, and can at least deduce that Illumi's childhood was similarly brutal to Killua's.

Naturally, the more interesting one that I find to be is Hisoka. It's amusing to wonder what it was that twisted the man's mind sour, or perhaps if he was born demented somehow.