It's hard to beat the author for analysis of one of her own characters. So let's start with Ms. Palacio for some general thoughts on Auggie's best friend, Jack Will. She writes:
Jack is a wonderful kid. He's not perfect. He's not sensitive like Summer is, but he knows what's right and wrong. He's not the kind of kid who thinks a lot about the things he does or its impact on others, but he's good-hearted. And courageous. He's the kind of person who will protect the underdogs of the world.
It's this combination of being pure of heart and kind of impulsive that makes Jack an interesting character. Between Auggie's unavoidable struggles and Summer's seemingly effortless morality, Jack—in some ways—is the most relatable kid in their group. He's the one who wants to be good but gets distracted by the easy road sometimes, which is behavior most readers can recognize in themselves.
When Jack's mom runs Mr. Tushman's request to be one of Auggie's welcoming buddies by him, his first response is, "Do I have to?" (4.The Call.15)—which is just an indirect way of saying he doesn't want to. At all.
Jack already knows how Auggie is when Tushman's request comes through, and he remembers the first time he ever saw August. It didn't go particularly well:
I know it wasn't cool, but I kind of went "Uhh!" when I saw him because I honestly got scared. […] It was the kind of "Uhh" you say when you're watching a scary movie and the bad guy like jumps out of the bushes. Anyway, I know it wasn't nice of me to do that, and though the kid didn't hear me, I know his sister did. (4, Carvel 2)
Jack's emotional reactions to Auggie's appearance—horror, fear, and ultimately guilt—all deter his interest in getting to know the kid. But that's just Jack checking out the easy road.
You know what isn't Jack taking the easy road though? His decision to put his initial reluctance on record. After all—Jack is the narrator when we learn about his initial reluctance. He could just as easily have not shared this with us or told us some tale about how thrilled he was at the prospect of helping a new student acclimate, but the kid has more integrity than that. So not only does he change his mind and come around to the idea of being on Auggie's welcoming committee, but he lets us know that he did so.
And what makes Jack change his mind? Somebody's got to protect Auggie from Julian. Jack steps up to make sure that Julian doesn't just destroy Auggie before school even officially begins. In other words, Jack knows bad when he sees it—and he's not going to sit idly by while it's unleashed on an unsuspecting new kid.
Even so though, Jack makes a deliberate choice on the first day of school to not sit with August in the lunch room, even though he knows Auggie will be eating alone. He says:
I had been hanging out with him all morning long because we had so many classes together, and I guess I was just kind of wanting a little normal time to chill with other kids. (4.Why I Didn't Sit with August the First Day of School.2)
Jack's motto at this point in the story is pretty much Do the right thing… if it's convenient. It's a better motto than Julian's, but it's still got plenty of room for improvement.
Even as Jack realizes what a great kid August is, he is reluctant to be friends with him. This is especially clear on Halloween when Jack is drawn into a discussion with Julian and some other boys about Auggie's awful face. During the conversation Jack says that he'd kill himself if he looked like August—and he explains away his relationship with Auggie, saying it was all Mr. Tushman's idea:
I mean, the thing is: he always follows me around. What am I supposed to do?
Not cool, Jack. Not cool at all. We think Jack wants to be a good friend, but he's got some work to do before he's good for more than fair weather.
Check out the epigraph for Part 4, which is Jack's section:
Now here is my secret. It is very simple. It is only with one's heart that one can see clearly. What is essential is invisible to the eye. Antoine de Saint-Exupery, The Little Prince.
One of the lessons that Jack has to figure out is that what he thinks he wants and what others think he should want do not always actually jibe with what he really does want. Specifically, Jack thinks he wants to be popular, and Julian thinks Jack should hang with more popular kids instead of with August. But here's what Jack realizes:
Why this is bad is because, well, (a) I don't actually enjoy hanging out with the popular group that much. And (b) I actually liked hanging out with August. (4.Ex-Friends.7)
The next two lessons Jack Will has to learn are (1) how to turn an ex-friend back into a friend, and (2) how to keep it that way.
It's gonna cost Jack big time, but in the end he isn't just going to get his dear friend Auggie back—he's also going to grow more comfortable in his own skin, and that's something no one can take away from him.