While the subject matter of Gantos's book is serious—daddy issues, a serial killer, extreme poverty—Jack-the-narrator maintains a pretty light and humorous tone throughout. He cracks lots of jokes and banters with most of the people he interacts with. Even when he's sneaking in to check on old people who might be dead (dressed as the Grim Reaper, no less!), he can always come up with a handy quip: "I'm the Norvelt Grim Reaper for the Public Good" (8.67).
But Jack is also thoughtful. And by this, we mean (no surprises here) that he does a lot of thinking. He's a curious boy, who asks a lot of questions, but he also spends a lot of time inside his own head, thinking about himself: "I knew I shouldn't read it, but I couldn't help myself. I loved to know other people's personal business. Mom called me a gossip lover. But I called it whisper history" (2.14).
As you can probably tell from that quotation, this thoughtfulness can sound pretty philosophically mature, like when Jack is in church thinking about imagination:
Real life was lived like doing a math problem: one and one always equaled two. But church had a different kind of math. You could never be sure what anything added up to, which meant that what was in your imagination while sitting in a pew was just as important as what the preacher was saying [...] It's like when you read a book and you know that the words are important, but the images blossoming in your imagination are even more important because it's your mind that allows the words to come to life. (13.88)
Pretty heavy stuff for a twelve-year-old. And Jack has had the nosebleeds in church to prove it.