MAJOR SPOILER
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In the novel it's shown that he grapes Juyeon I want to say in middle school or high school with a couple other guys, records it and then it ends up getting sent to his mother which is why she ends up in a mental hospital, but I want to say Jiwook loses all his memory ( it might be just some) so he has no idea that the reason Juyeon has him trapped is for revenge

???