yjh is affected the most by kdj's last sacrifice, i guess.
he had been regressing for countless times and being alone for hundreds of years and suddenly kdj popped out of nowhere, screwing every scenarios he had been through, but then trusting that rat-bastard to be his life and dead companion, furious when he did self-sacrificing and not appreciating his own life, looking up for him for 2 days when kdj in deep sleep post demon realm scenario, decided to not regress for kdj, finally giving all his trust to him, wanting to understand, but hadn't given the chance.
after saying he would not regress anymore, he did regress again, trying to save kdj.
the scene when yjh was alone with kdj fragments in space for days hits me hard. damn. yjh finally learned about kdj; how did he lived before scenarios, his past, his buried feelings, and why kdj didn't though about himself much and those persistent self-sacrifice...
yjh read kdj's life repeadtedly. just like how kdj read his entire lifetimes through WoS, trying to learn and understands him, yjh feels the same way as well.
through kdj, he was saved. hence, yjh also wanted to be the one who saves him.