Green narrates from Miles "Pudge" Halter's perspective, and this is both awesome (we get to hear his thoughts on everything from girls to friends, no holds barred) and not so awesome (he's kind of a wreck after Alaska dies).
As a newbie to the Creek, Miles views everything from the perspective of an outsider. He has to describe everything, give the backstory for everything, and navigate the social and academic ins and outs of the school. As readers, then, we're equally as awkward and new to the Creek as Miles is, and so its perfect that we get to see the place—and get comfortable in it—alongside him.
Because of the nature of first person narration, we feel mostly on Miles's side during conversations and conflicts—when Miles gets hazed by the Weekday Warriors, we get mad; when he feels like he's made his first friends, we cheer. That's part of the nature of having a sympathetic first person narrator. We like him, and we want to see him overcome whatever it is he has to overcome. He's awkward most of the time too, which makes him easy to relate to. Add his general frankness to the mix, and Miles becomes a pretty credible narrator.
Which is why when Miles becomes less reliable as a narrator after Alaska's death, it throws us for a loop. He's a pretty self-aware guy, it's true, but because of the powerful emotions he feels, he gets a little lost in his emotions and loses his grasp on reality. Check out what the Colonel has to say about this:
"Do you even remember the person she actually was? Do you remember how she could be a selfish b****? That was part of her, and you used to know it. It's like now you only care about the Alaska you made up." (13after.35)
The Colonel calls it like he sees it here. And the thing is that we know that Miles recognizes the flaws and faults in Alaska, because he comments on them at the beginning of the novel:
Her "sweetie" felt condescending, not romantic, like a boy enduring his first biblical rainstorm couldn't possibly understand her problems—whatever they were. It took a sincere effort not to roll my eyes at her. (84before.15)
But after she dies, he forgets this side of her, he forgets his friends and pseudo-girlfriend Lara, he forgets everything but his staunch belief that he loved Alaska and she could have loved him. The way he misremembers Alaska is a key part of what skews his perspective.
Because of his unreliable perspective in the second half of the book, we have to look for insight in other characters, which only occurs when Miles is talking, thinking, or acting near them. (Curse you, first person narrator.) The Colonel reproaches Miles a few times, and Takumi gives Miles a gentle nudge or two in reconciling with Lara, but it's only when Miles lets go of his desire for an Alaska who never existed and forgives both himself and his dead friend that he's able to regain his balance and reliability as a narrator.