I won’t give any specific spoiler but,
*Spoiler *
The other ‘survivor’ is actually just sleeping, they told the MC beforehand to wake them up if they are falling into deep sleep (because of his skill), but the MC just got so shocked and just forget about it and the rest is I guess you better read the webnovel. In the end this ‘survivor’ helped the ML to help MC in the great dungeon final battle.
yeah, they kinda sleep, I mean they sleep like dead person thats why everyone thought this person was dead for sure. And the MC is not on the spot when this ‘survivor’ dead, so the MC just got so shocked ( I think the MC wanted to revived the body of this ‘survivor’ but other revengers said that it wasn’t possible and they need to get into the next stage asap) . And the only one that will meet this ‘survivor’ is just the ML. But who knows in the future, because the ending is like open ending where their journey to the galaxy just begins so there will be possibilities they will meet again, but sadly the ending of the web novel is the beginning of their journey.
I was thinking here, only 5 survived in the great dungeon 10 years ago, but only 4 left the portal, Chaeyoon said that Kangjin the other survivor was missing, we also know that after defeating the boss a black portal appeared, so what if Kangjin stayed back in the portal using the skill that stops time/decreases the passage of time in the black portal to prevent a breaking dungeon from occurring all this time and that's why he hasn't left the portal and is missing.