I don't know anything at all but from these hints dropped, this is my theory. I don't think Cain is the bad kinda sus yk.(and fuck Yahwi)
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My theory is jooin and Cain had some sort of connection in previous life and jooin showed kindness to Cain. And Cain thinks of jooin as his master and tries to return the favor to ensure he's okay in this life and loves him truly.
Man I thought this manhwa was dark, had a lot of mystery to offer.
Just turns out to be another one with an obsessed stalker psychopath. Nice.