My best bud, Acheron, once explained it to me like this. (Prologue.4)
When you first read the book, you care about what Acheron has to say about fate and free will. But thinking back on this quote, you realize that Nick is calling Acheron his "best bud." We're not sure when exactly they'll become friends, because they only hang out once in this book. It looks like they reach BFF status eventually, even though many people warn Nick to say away from Acheron.
"I just wanted to say that you should be very careful with what you learn from others and who you let into your life." (4.36)
This is good advice for anyone, but it's even better advice when your friends might be demons from another dimension.
"He's the best friend you'll ever make or your last enemy." (7.74)
This is said to Nick about Acheron. And by the end of this book, we're still not sure which Acheron is: best friend or enemy. Nick calls Acheron his "best bud" early on, but he also wishes he'd never met him. Maybe they're more frenemies than friends?
"My enemy's enemy is my friend." (9.36)
With demons, vampires, and other monsters on the loose, friendships can feel more like alliances against a common enemy. Is that still friendship? Maybe the beginning of friendship?
"What are you?"
"Your friend. Always. I'm the only being you'll ever be able to trust."
Bullcrap. The only person he could fully trust was himself. (10.52-10.54)
This little exchange is ironic. Nick says he can only trust himself. What he doesn't realize is that "Uncle Ambrose" is he himself from the future. So, yeah, he can trust Uncle Ambrose, because they're the same person.
"I'm on your side, Nick. You don't have many friends, and even fewer you can trust." (10.65)
Caleb is right when he says this to Nick. Although it again sounds like they're talking about being business partners instead of friends, they're one and the same when you're side by side facing off against zombies.
"In the case of Caleb, he's your protector. He would die to keep you safe." (16.6)
In war situations, even if the war in question is against a gaggle of zombies, brothers in arms become the best of friends. What is it about a tense situation like this that makes people bond more quickly than they would otherwise?
"It's why some of us haven't been real welcoming to you. We don't like being with mundanes who don't know about us. No offense." No offense? Most of them had been royal jerks to him. (16.131-16.132)
This is a flimsy excuse from some of the popular kids about why they've been cruel to Nick. While they won't be his enemies now that Nick knows their secret, we can't imagine they'll all be friends, either.
In the end, nothing could be crueler than leaving Stone to live his putrid life of false friends and petty jealousies. Friends who didn't really like him. Who only wanted to use him for what they could get. (18.9)
Nick makes a very astute observation. Popularity isn't important. What really counts is having friends who genuinely care about you, not the number of people you have in your social circle.
His enemies were the ones who told him lies under the guise of being his friends. The ones who wanted him to be like them. To ruin his life and throw away everything he'd worked so hard to become. (18.28)
We're not sure who Nick is talking about here, because he hasn't yet been betrayed by anyone who says they're his friend. But it's good that Nick realizes that this is a possibility in order to keep it from happening in the future.