I feel like people who say "we don't understand YJH because we're stuck in KDJ's head" (because many people actually have this mindset) are missing something kind of very obvious.
Being in KDJ's head isn't a limitation—it's the opposite, actually. It's the whole point. He's literally the one person who's been paying attention to YJH for over a decade before the story even started. He notices the micro-expressions, the patterns, the things YJH never says out loud. He reads his fucking thoughts. He's constantly observing, correcting his own assumptions, and learning to see him as a person instead of a character. And yeah, KDJ gets things wrong, no shit. He admits it himself at one point. But he keeps trying. He doesn't just settle on a surface-level reading and call it a day. He keeps paying attention and keeps adjusting his understanding and realizing he was wrong and trying again. That's literally the novel's whole thing about false dichotomies. It's constantly showing you that the lines we draw between "real" and "fake", "character" and "person", "I understand" and "I don't" are all just lines in the sand. They're meant to be crossed.
Like, people will say YJH's deaths are "off-screen" or "filtered through TWSA" as if that makes them less real, when we're supposed to feel the weight of them through KDJ—the guy who read every single one of them for God knows how many times. The guy who knows the number, knows the details, knows how much it destroyed him just to read about it. So if you're reading from KDJ's perspective and you don't feel the devastation of YJH's deaths, you're not reading KDJ right. He's carrying that grief the entire time. It's not "on the sidelines", it's fucking built into his bones. Also, the 1863rd round exists. We literally see YJH's spiral, see him split himself in half because he just can't handle the pain clawing inside his ribs anymore. We see him give up. We see him turn into something else entirely. The novel is explicitly showing you exactly how broken he is.
Saying "we don't see YJH's deaths" is just not true, you're just refusing to look. We see the aftermath, we see the cumulative damage, we see the person he becomes after dying 1863 times (speaking of which, no one seems to understand the relationship between KDJ and SP and just crumples it into something reductive, but I'll cover it later). That's more impactful than watching him die on-screen for the nth time. The novel trusts you to understand the weight of that without showing you every single death, but apparently that was a mistake.
And it's not even like YJH's near-death moments are hard to find. The obvious ones that immediately come to mind as great examples are when he tried to sacrifice himself to become the 73rd Demon King to save KDJ from having to do it, or when he got burned alive by Surya after standing against him defending an unconscious KDJ. He was ready to die so often during the story, not because he wanted to, but because he thought his own death wouldn't matter as much as KDJ's. That's the self-destructive mindset people pretend doesn't exist because it's not as funny as "lol KDJ dies again".
If you're reading from KDJ's perspective and still don't understand YJH, that's not because his POV is just so unreliable or wrong or whatever the fuck. That's because you stopped paying proper attention to the narrator you're reading. The novel gives you everything you need to understand YJH even if you personally can't relate to getting yourself killed way over a thousand times (neither can I, who'd have guessed??). It's just that understanding him requires you to actually engage with what KDJ is observing and recognizing the patterns instead of expecting everything to be spelled out in bold letters.
People say readers relate to KDJ because he's the center of the story and everything revolves around him. And yeah, that's true in a narrative sense, but the fucking thing is that KDJ himself revolves around Yoo Joonghyuk. His whole existence, his motivation, his reason for even being in the scenarios. It's all tied to YJH. He built his entire identity around him. So if you relate to KDJ, you're relating to someone whose entire orbit is Yoo Joonghyuk. You can't claim KDJ as your self-insert and then ignore the person he's built his whole world around, that's not how it works.
For a fandom built around a story about reading, some people really don't want to do the reading part.