All of these complaints... But it's actually quite good.
What I love is that it's clear that there are many different stories. Jun Pei is the protagonist -- of his own story. Everyone else has their own story as well, some greater, some lesser, some equivalent.
Noriko makes her self-serving choice, and then very quickly ends up being abused, then raped, tortured, mentally broken. The crux of her character is that she's not a bad person, but also not as good person as she initially thought that she was. Which makes her a normal person. I think the worst thing for her was the too-late realization that the best choice would have been running away with Jun Pei, in which case they would have had a good life (maybe). And while she claims to be trying to "save" Jun Pei by killing him, more likely she's just trying to erase the reality and guilt of what she did to him. And yet still, in the end, she does apologize to him -- which she did really want to do. (People can have contradicting motives.)
Mugetsu's story is much longer (thousands of years) and likely much more epic than Jun Pei's. Jun Pei's story is basically just a side-quest for her. We know that story is there -- even though we see only the tiniest fraction of it.
Gocchan/Lihelt must also have a hell of a life (being part of the demon slayer party for one). Here we only see the bits of it where it intersects with Jun Pei's story. Again, we only see a bit of the story, and see that it turns out happily.
Kido has own story. He does leave behind his past and start a decent life. But then his old sins catch up with him, and it's not just him, but his wife and children that pay the price for what he did.
Sakagami, the guy with Clairvoyance who came to rescue Noriko, had his own determination as well, and worked hard to reach the point where he could save her (only to be offed by the broken Noriko). Again, a man with a story of which we only see the tiniest part.
The Letter guy: Again, we get a bit of his story, and that sounds fairly epic as well.