One reason Infinity is so relatable is that we've all asked ourselves the questions addressed in this book. Questions like, "What is it like to struggle with extreme poverty?" Or, "How would we survive a zombie apocalypse?" Or, "Could we ever be friends with a half-demon who is being blackmailed by our demonic father?"
Caleb literally appears out of nowhere in Chapter 9, and Nick starts talking about him as if he's been around since the beginning of the book. It seems that Caleb goes to school with Nick. Caleb hasn't been mean to Nick, but they're not exactly friends. Nick isn't sure if he should trust Caleb or not, which is another good question to ask yourself about anyone who seems to appear from out of nowhere in a dark alley.
Nick and Caleb soon become friends because it's clear Caleb has Nick's back. However, we eventually find out that Caleb is an old demon who has been sent to keep an eye on Nick. Ambrose tells Nick, "Do you think he's enjoyed being in high school with you and the others when he didn't have to be?" (16.8).
Caleb is basically the 21 Jump Street of demons—he's basically going undercover as a high school student—but it seems he has grown to care for Nick while observing him. What Nick doesn't realize is that Caleb has been sent by Nick's own demonic father, and we're not yet sure what his true motivations are for wanting to keep an eye on his boy.
At the end of the book, Nick thanks Caleb for helping him. That small gesture is a huge deal to Caleb. "Not once in all these centuries had anyone ever thanked him" (Epilogue.8). Hundreds of years is a long time to go about a thankless job, so maybe these two have a future in friendship after all.