i read the novel back in 2020 and i reread it everyyear. spoiler for others
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tbh there wasnt ever any emphasis on kdj revealing his face in the entire novel. like no particualr scene where they suddenly started noticing his face. and the thing you brought up abt yoo sangha and HM makes me also wonder. there is this scene in the dark castle
chapter 152-
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Don’t just skip to this part. It’s better with the context, and it’s not really the reaction you’re looking for without it.
“That person is in hell. Quickly forget that ugly face."
Shin Yoosung showed an unexpected response to her harsh words. "I really don't understand. Everyone says he is ugly but how is he ugly?"
The question was a bit embarrassing. In the meantime, she had pushed 'Kim Dokja is ugly' like a slogan (the constellations sometimes gave her coins) and she never really questioned why Kim Dokja was ugly.
"I mean, the shape of his eyes and nose… the whole harmony…”
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The more she talked, the more confused Lee Jihye became. She couldn't understand where Kim Dokja was ugly. No, Kim Dokja's face didn't come out clearly. It was as if it was obscured by something hazy…
Or like a face that hadn't been created yet… Why? Why couldn't his face appear clearly?”
it makes me wonder if his face was blury entire life. or maybe yoo sangha sees his real face and is confused when people call him ugly. it can be anything. in this scene, i think shin yoosung could maybe get gist of his face because she is her incarnation. i mean the entire reason his face apears blurry is because of the fourth wall seprating him and everyone or (spoiler)
yk when kdj starts talking abt three ways of surival, his words get censored because it is related to the end so just like that kdj face is censored because he is related to the end.
(there are different theories)
so it would make sense for people to start seeing his real face bw 510-551 chapter.
conclusion : since the author never made a particular scene neither explained it, its upto us to interpret it how we think. i mean the entire novel's point is that.

When will they have a glimpse of Kim Dokja's real face? It's frustrating when they think he's ugly,when Jihye saw his real for the first time she's like damn he's fucking handsome