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Since there was no one supporting Sa-young, Ga-young who was the woman in charge of him, told Sa-young that she was gonna move him to a different hospital and that if he followed, he'll be able to meet J.
Since J was mentioned, Sa-young agreed. And so, he got experimented on everyday for almost a whole year. Then he awakened one day, melted the facility, and he got saved by Jungbin and Wonwoo (in the manhwa flashbacks)!
Ga-young lived though. And later in the novel, J gets to meet her and oh boy J was LIVID.
This is a spoiler!!!!!!
but the lady who was reporting to J about Sa Young's condition in the latest chapters convince Sa Young that J will come back if he goes with them or he will gain the power to protect him. Just mainly he was manipulated into agreeing to go with them because of his desire to see J. Then tortured with poison with the powder that produces awakeners.
I think. Its just bullshit.
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He really wanted to kill her but at the time, there are more pressing issues (aka the apocalypse) and Ga-young unleashed one of her experiments who had powers EXACTLY like Sa-young's to fight him. Btw, Ga-young doesn't know that Sa-young was the kid she experimented on lol
Near the end, J asked Sa-young if he got experimented on the same as her other experiments (bc J found out their other labs and how cruel the experiments are) and was really pissed but Sa-young stopped him saying that he didn't have to get his hands dirtied.

I kinda forgot some parts of the plot. What happened after J left young Sa Young? Did the hospital send him to Promethus? Or was the hospital the Promethus?