He might only make an appearance at the very end of the book, but Kristina's son, Hunter, is still a big part of the aftermath of his mom's bad behavior. The child that results from her rape by Brendan, Hunter embodies both the best and worst of his mom's new life.
On one hand, Kristina describes him as a second chance that she believes can transform her; as she says of his birth, "Suddenly, for the first time in a very long time, everything felt right" (The #1 Best Thing.6). Kristina says her son is beautiful and is grateful for her decision to give him life rather than go through with the abortion.
On the other hand, though, Hunter also serves as a grim reminder of the bad decisions and events that led to his birth. For one thing, he's still Brendan's son. For another, he displays many of the symptoms common to babies born to drug-addicted mothers. "He cries a lot and he doesn't really sleep like a newborn should," Kristina says. "No lectures, okay? I accept my part" (Happy Endings.2). Clearly, Kristina takes responsibility for the mistakes that led to these issues and is willing to love her son in spite of them. Fingers crossed she can stay clean for him.
We'll have to read the sequel, Glass, to find out exactly how Hunter develops and changes his mom's life.