As much as Jinu gets glaze from one side of the fandom and the hate he gets from the other, I think he is a pretty good example of a well-written morally gray character. Like, a true neutral imo.
Sure, he was manipulated by Gwi-Ma. But going out of his way to make the Saja Boys was not Gwi-Ma. It was all Jinu's idea. As the demon king himself says "In 400 years, you've never done a single thing that didn't serve yourself". Forming the Saja Boys wasn't in Gwi-Ma's interest or the world's interest, it was purely for Jinu's benefit. If Gwi-Ma gained from it, that was just a byproduct.
Also, Jinu was willing to kill innocent people for what he wanted, erasing his memories. As seen in the train scene, the bath house, etc. We don't know how many more were killed off-screen. He is a mass murderer by proxy.
Jinu starts to have a "redemption" midway through the movie. We learn about his past, his sister and his mother, growing up in poverty.
He was in a bad situation, starving, a peasant, and he made a deal with the devil. One that left his family alone, that made him a performer for the king. He lived a comfortable live, leaving his family alone. He did a bad thing for a good reason.
Rumi believes he can change. They sing an entire song about escaping from their past and their insecurities, being free. He promises to let her win the idol awards. Then, he can finally be free from Gwi-Ma.
Then Jinu fucks it up again by betraying her. He was manipulated by Gwi-Ma. It does explain why, but it doesn't make it right. He hijacks the event, pretends to kidnap Bobby to lure Mira and Zoey away, and sends in fakes of them to reveal Rumi's deepest, darkest secret. The secret she has only told him. He reveals it in front of everyone. He got to know her, her secrets, her deepest fear, and used it against her.
Rumi has a mental breakdown and gets weapons held to her by her own friends. These things upset her so much that she literally begged her own mother (figure), Celine, to end her life. She thinks she's as worthless as a demon. That's all she deserves. The day before, with Jinu, she probably felt better than ever about herself. Because of his actions, her self-esteem was destroyed to the point of suicidality.
Rumi then has to clean up Jinu's mess. Gwi-Ma attacks Rumi, and she likely would have been hurt or died if Jinu didn't save her. His act is heroic, but he only has to sacrifice himself because of things that he caused. Without Jinu, there is no Saja Boys. Without the Saja Boys, Gwi-Ma would have died or been weakened to the point that it never happened.
While there is nuance to the idea that if the Golden Honmoon did happen, Rumi would have been killed because it "cleansed the world of demons", the way the movie plays out is still mostly Jinu, and moreso Gwi-ma's fault. If he had followed the plan, he never would have died. But also, he was tortured to make sur he didn't. Because he fell victim to Gwi-Ma, the whole movie ensued, and he sacrificed himself to save Rumi.
I honestly don't think the sacrifice was enough to fully redeem him. Jinu can't take back the harm he caused to Rumi, the people he killed, the fans he manipulated. He will have to do the work in the second movie for me to forgive him.
I do think Jinu is a good example of manipulation, though. His actions are his own, though they were heavily motivated by Gwi-Ma's psychological torture.
This post is getting long and rambly now. Jinu isn't a favorite character of mine by any means, but I do admit he is a well-written morally gray character.
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