I respect your opinion even though you put aside some important things.
First, it's not his fault that he looks like that, and I don't think Luan finds him attractive because he looks like a child, I think he just like him and his personality.
Second, as you said he was in isolation for all his life, so much that he had to drop our of school, but... This means he never learned how to take care of himself, neither did he learn to want to take care of himself, because the only person he saw was his nemesis, you can clearly see that Yul has depression and that he thought about throwing himself off the building multiple time throughout the story. Depression is a hard thing, as someone who's been through it, I can tell you it's hard to accomplish anything, some people just stop completely taking care of themselves, no shower, no teeth brushing, no eating, no drinking, no sleep sometimes, even.
Third, I know that this is just a fictional character (😂) and that the author decided to draw him that way, but there are many reasons why he could look the way he does. For one, he might have an illness that no one knows about since his Hyung don't take him to hospital (that's just a guess though). For exemple, I take Vyvense (ADHD medication) and it made me almost starve since I was never hungry, I had to take supplements as a child, and I was never as fat or as tall as I should've been.
I'm telling you this respectfully while your opinion is your own to have :) I'm just saying that judging someone from their looks is always bad, because you never know what someone might have gone through to look like this, and love is blind after all, no? Have a great day 🥰
