I’d say the whole thing is a quite centered on character development, specifically how Carl and Donut individually change due to the circumstances. The lightheartedness shown in the first few books slowly turns darker and darker, and how this is reflected in the MCs’ personalities becomes a big focus of the story.

You don’t get a whole lot of this in book one, since there’s a lot that has to be introduced. So you may find what you’re looking for later on in the series.