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There's one vital thing Hyehan can't bring back-- a soul.
He may be able to revive a body, but the thing that makes us truly alive is our souls. He was already long gone by the time Hyehan found him. If Charin does somehow manage to be revived it's not gonna be the real him. He's basically going to be a vegetable or worse a zombie that only knows how to act on the basest of instincts.
Then there's the feeling I get that if Tehir's tissue/organs/DNA gets mixed in, like Hyehan is trying to do to revive Charin, it's gonna result in the two of them merging into one. They already look shockingly similar, and given their shared parentage and biological dispositions it wouldn't be entirely shocking if they could become one person. The problem is tho is that Charin's body is an empty husk. It's more likely that Tehir would probably take over once his organs have been transplanted and thus giving him a new body that isn't actively killing itself. I don't think I like that idea too much lol
And I'm not much of a hopeless optimist to think Charin could come back wholly as himself and he could finally live happily with Jin. So, imho, Charin staying dead and Jin learning to move on and become his own person is the best outcome for the two.