Oh, Agnes. You're kind of mean, and we really wish you'd learn to think for yourself a little bit more, but you're the only narrator we've got, so we're sticking with you.
In Nothing, you have to think of characters more for what they represent than who they actually are. Agnes represents the group, or, in Kierkegaard-speak, the inauthentic individual. She could really be anyone; she's interchangeable with the others. And that's precisely the point.
Unlike Pierre Anthon, Agnes is afraid to climb a tree and admit that nothing matters. She's totally invested in being somebody and amounting to something, and in this hang-up she becomes a sympathetic character—or at least as sympathetic as anyone is in this book. After all, most of us know what it feels like to be totally invested in who you think you're supposed to be. And we'll give her props for going and getting Pierre Anthon and bringing him back to the sawmill to try to save Sofie, even if it does lead to his death. In trying to do the right thing, she redeems herself (at least somewhat) for making Gerda sacrifice her hamster.