OKAY. I honestly ask myself this question everyday. I have to admit I'm biased because I find Nagito extremely attractive, but I am going to try to explain why I do so.
I'm absolutely obsessed with anti-villains. He's a perfect example of it. He also kinda fits the Byronic hero stereotype which is my favorite trope in the world: this tormented, highly intelligent, charismatic outsider with a dark past and self-destructive tendencies. His character is based on the contrast between many aspects of his life: his lucky-unlucky cicle, hope and despair, his sudden changes in personality and the way he treats people. I love how he seems so unpredictable and crazy yet has such consistent and strong ideals. It's also the contrast between his "bishounen", beautiful boy, pure and innocent appearance and how fucking nuts he actually is.
I think part of what makes him so attractive to people is how servicial he is and how he treats the others students as superior (the writers knew exactly what they were doing when they wrote him to be a servant in UDG). His self deprecating and masochistic tendencies are really hot, especially because HE'S SO DAMN INTELLIGENT AND BEAUTIFUL AND DOESNT EVEN REALIZE IT.
Setting his attractiveness aside. He's such an interesting character and a perfect villain for a sequel. In THH, Byakuya torments everyone for his own selfish desires. Nagito torments himself for everyone else's. He knows that in the situation they're in, people need a common enemy to stick together and he is willing to sacrifice himself to be that.
Personally I don't relate to him at all except for the fact that he's a control freak lmao (I don't understand how someone could ever relate to him and I am scared of his kinnies)
Even after saying all this, I feel like I can't quite put my finger around why I love him so much. He has and aura of mystery about him... You never know if he's a good or a bad person, if his actions are justified, and it drives me CRAZY. I love discovering new things about him and understanding him a little better everytime I replay the game.
In conclusion, I think it's that duality what makes him so charming to most people. I tried to keep this as short as I could and it's really messy but I'll probably write more about it, or make a video essay or something lol. Thanks for liking the art and asking the question !!! ❤️