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I don't exactly remember their reactions after their memories come back. However, I believe Junseo and Sehyun went to the hospital when they get notified of Junhyuk's accident with the truck (he survives). They learn of Seohan's death as Seojun came to the hospital with Seohan who was in critical condition but died in the operating room. The three of them were in the same waiting area.
For Ian, I think he heard the news somewhere since Junhyuk's accident was reported by the media. I think Haim was in the same room when the remaining members of Stardust got the call about Junhyuk's accident and later learning of Seohan's death. Seongbin hears it later since he left the group due to his mental health. Finally, Siwoo was actually at the site of the accident just like Seojun while he was visiting South Korea after living an ordinary life overseas. For Siwoo, he wouldn't have recognized Seohan who got hit by the truck, but his current self recognized Seohan and was devastated as he could only watch during the memory replay. In addition, the members remember some of the lives they led before Seohan's death like how Stardust fell apart or how Ian and Siwoo did not become idols like Seohan.
Overall, every member was shocked about Seohan's death in different ways. They were also suspicious about Woochan's intentions since they didn't know if he was actually an ally. After remembering the past life, they all become overprotective of Seohan since he keeps collapsing when each member remembers one by one or gets into accidents caused by the world trying to erase his existence.
Thank you (≧∀≦) btw do you remember which chapter is it?
Unfortunately no, but it does take place over a big part of the novel since each member gradually remembers little by little, even Seojun reaches 100% sync much later after a few members reach 100% sync. However, with each person who gets 100% sync, they help Seohan and Woochan with completing the quests from the system window.
Question! You said the world keep trying to erase seohan's existence.
Was it shown why seohan must die?
And also why the system helps seohan (and woochan) survive
can you please send me the novel link if you have it?
Please spoil me every reaction to Seohan's death and their reaction to Seohan once they remember their parallel self memories