I dont remember the story in full detail because I read the raws when this story first came out, but I'm pretty sure this is what happened
SPOILERS AHEAD
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I think Kister and the MC were actually dating before all this but then Kisters twin or brother or whatever began murdering people and he also liked MC, but Kister felt bad for the brother or made a deal or smth then "turned himself in" and then MC lost his memories because of a traumatic event that I do not remember very well (sorry) and then the twin got caught or whatever and then Kister got released so it ended up being a happily ever after :) (sorry for any grammatical or spelling errors, English isn't my first language)
Just read it all in korean and let me tell u... (spoilers lower)
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OHMYGOD I loved the last words- brought me to tears. But I am still infuriated with Jonathan not running straight to Kister, I don't think I've ever been so sympathetic towards any character as him and he deserved to have somebody waiting there at the gates and after the play. Goddamit I really don't like Jonathan, first not having a single care about his brother dying and now not taking proper care of Kister outta prison, love him already dammit he deserves all the love in this world! (╯°Д °)╯╧╧