Jinu's main arc is from the worldview that good and evil is something you are, to realising that good and evil is something you do, something you choose each moment. At the beginning of the story, he (believes he) did something evil, therefore he is evil, and that's all he'll ever be and do. Rumi's existence challenges that worldview, and her passive inspiration and active interventions (including at least two magic spells) help him realise that he doesn't need to let that one moment define him
In the finale, he had thought Rumi destroyed, turned to demon, yet there she is, scant hours later, her right hand still a claw, squaring up against Gwi-Ma, on the human side
An earlier key moment in his healing comes when he owns his guilt, from "you left them" in Gwi-Ma's voice to "I left them" in his own voice
In the bathhouse, he is shocked by the revelation that someone can be a hunter and part demon
On another layer, he has his part in the "enemies to lovers" trope
On yet another layer, he plays yin & yang with Rumi; at the beginning of the movie, they are each part demon, part human, in conflict; Rumi wants to resolve hers by erasing her demon part, Jinu by erasing his human part
(footnote: it's pretty clear that Jinu didn't willingly abandon his mother and sister, but that's not really important to his healing arc)
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