Katz is an odd bird. On one hand, he can be so annoying that you might roll your eyes every time he appears on the page. On the other, he proves himself to be a pretty solid—not to mention funny—dude by the end of the book.
In order to make sense of this disparity, we'll need to take an in-depth look the man himself, Little Debbie's Number One fanboy.
Bryson and Katz go way back. They took a trek across Europe together when they were fresh out of college (which Bryson documented in Neither Here nor There), but it wasn't exactly a swell time. Katz, being Katz, was as annoying as a tone-deaf karaoke singer. Though they "remained friends in a kind of theoretical sense," their "paths had diverged wildly" in the years since that formative experience (1.2.19).
Frankly, Katz' life hasn't been going too great. Unlike Bryson, who enjoys an idyllic existence in New Hampshire countryside, Katz lives a humble, working-class life. He's not married, and has no kids. What's more, he's recently sober, having "not touched alcohol or an illegal substance since" going to jail five years earlier for possession (1.2.50). All in all, these small glimpses into Katz' life suggest that he's coming to the AT to escape his real life.
After all, it's not like the dude loves hiking or anything. He complains constantly. He repeatedly tries to quit so he can binge-watch episodes of X-Files (not that we can blame him). And let's not even get into the Little Debbie fiasco. Basically, if Katz is having a good time on the AT, then he's delivering an Oscar-worthy performance fooling us. True—there are plenty of moments when he buckles down and gets down for business, but he rarely seems to enjoy it.
This attitude frustrates Bryson until he gains a better understanding of Katz. In fact, it's not until Katz falls of the wagon that he tells Bryson,
"It's just that sometimes all I see ahead of me is TV dinners—a sort of endless line of them dancing towards me." (2.20.25)
In other word, dude's lonely. It's at this moment that Bryson realizes how much their friendship means to Katz—not to mention how much it means to him. They might not hang out every day, but they'll always be there when it counts.