I haven't completely finished it all yet but so far right now all I can say is [Heavy spoilers about the relationship between Sa-young and Eui-jae]
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As soon as he confirmed Eui-jae was J, he scrutinized his Eui-jae's character and pinpoint a way to hold him down from recklessly burning himself out: becoming the shackle to Eui-jae, someone who is deeply riddled in guilt for being the sole survivor of the West Sea Rift, and becoming the person Eui-jae could return home to
It worked like a charm because of it, Eui-jae's survivor guilt, because Eui-jae's desire isnt a "peaceful" life, he actually wants to atone. He wants to atone for the sin of surviving by properly protecting the people around him, he wants them to live. He doesn't want to be alone, he wants someone to go back to and who would wait for him to come back
And that's what Sa-young is for him, someone who will wait for him
The reason Sa-young has been searching for "J" is because Sa-young is the boy that "J" had once saved in the past years ago. Sa-young was the boy "J" had always come back to in his lowest of times. Sa-young is the person that has always been waiting for him to return, both then and now..
just came back from reading the novel and I can't look at Sa-young without feeling sad anymore...