Writing for Killua is such a balancing act. He's got this intense, violent upbringing layered with a genuinely sweet and loyal core he's discovering. The challenge is making the 'reader' character believable enough to earn his trust without making them a Mary Sue who magically fixes all his trauma. He's not a typical soft boy; his affection shows through action, through choosing to stay, through protecting someone even when it terrifies him. I've read so many fics where he's softened into just a cute, blushing anime boy, and it strips away what makes his character arc so powerful. The best ones I've found keep his edge—the casual references to assassination, the moments of chilling practicality—while letting the relationship be the safe space where he learns he doesn't have to be a weapon. It's a slow, fragile process. You can't rush it, or it feels fake.
Another huge hurdle is the power dynamic. Physically and in terms of life experience, he's vastly more capable than any realistic reader insert. The author has to navigate that without making the reader feel useless or turning them into a helpless damsel. I've seen clever takes where the reader's strength is emotional intelligence or a normalcy that grounds him, something the Hunter world lacks. But if you get it wrong, the whole dynamic feels off, like a god dating a mortal without any of the interesting mythological consequences. It's why I often prefer fics set post-canon, where he's had more time to just be a kid and the gap feels less extreme.