So...I've said this last chapter, but I feel like it needs repeating again:
It's not hard to see why people still rag on Miya, but I look at it this way.
When you think about it, Miya is expected to be sad, cry and grieve over her mother's death because she's a young teenage girl, so she'll undoubtedly have a lot of shoulders to cry on and gain support from her classmates and teachers. Kosuke, on the other hand, is expected to shoulder all the criticism and responsibility to take care of her, work to keep a roof over their heads and food on the table, and interact with her like everything will be ok even when he's feeling the exact same way as she does. If she wants to just up and run away, she's pretty much allowed to, and who has to shoulder the responsibility to try and bring her home and make sure she's ok? Him.
It's not that she's a selfish brat. I find that it's more that she's a bit too wrapped up in how she's feeling that she fails to see that he's also suffering as much as she is, with the main difference being that she can get away from it all while he has to stand and fight, even when he's out of weapons, ammo, and armor. We also can't forget that before Koto, he was getting virtually zero support from anyone during the early parts of the story. Even the most well adjusted adult would start to crumble underneath that kind of neglect.
But even with all of that said, I can't blame her for not helping him. She didn't know how much he was suffering, and that unintentional neglect unfortunately was a major factor that lead to the current situation.
Now to add on to this, what Kosuke's doing is wrong in plenty of ways. I don't support what he's doing in the slightest, but we still can't forget that Koto was the only one who helped him in his time of need, so it's somewhat not that difficult to see why he's chasing after her even though it's wrong, so for these types of issues, it's more about understanding the reasons why something occured, not agreeing and even justifying the reasons why something occured. I think that's the confusion and the reason for the haters/defenders of Kosuke here. As for Miya trying to understand him...I'd actually argue that the kouhai had more time to understand Kosuke than Miya did. All she kept doing was running away when things got bad and only "tried" to understand him when she felt like it. Again, I can't exactly blame her all that much due to her age and mentality, especially with the whirlwind of events happening around her, but even she has to realize at some point that she's not the only one who's hurting, and Kosuke's situation escalated to this level partly due to her unintentional neglect.
Now, I don't think this story is supposed to have a family friendly ending. It's really about two people who found hope (and love) in each other at their worst moments of their lives when others did not. The ages only add to the drama and everyone around them reacts in a realistic way, but the two main family members, Miya and Koto's mom, also fail to realize their unintentional contribution to the whole situation.