Every single character from minor antagonists to major protagonists have a deep backstory, beautiful design, and, most importantly, a truly marvellous character arc. Every character is so deep, they are like real people, whom value aspects of society and relationships.
Each story arch is beautifully constructed with their own well thought out plot, and fantastic antagonists. Honestly, some of the 'bad guys' were my favourite character ever.
While this is the longest anime I've seen, it certainly didn't feel that way Nearly all other story's I have encountered have dropped off and made me bored at times, and at those times I have dreaded picking up that story again. HxH has never done so, I have always been fully engaged with the story and it's characters. Even character's I expected not to like turned out to be really great.
From Killua and Gon, Kurapika and Leorio, Knuckle and Shoot, to the Phantom Troup, Hisoka, and Illumi, I was captivated by each and every chartacter.
