Down to Earth / Colloquial
Here's the thing about Turtle: She might be cynical and judgmental when it comes to just about everybody, but she's also really down to earth. She calls it like she sees it, and since this book is written in her words, the writing style is down to earth, too. Check out what she says about the treasure map and coin:
I go back and look at them every chance I get. I keep thinking that maybe they are real. It's like monsters. You know there's no such thing, but you can't help but wonder if they're out there somewhere in the dark night, just waiting to get you. (13.23)
There's something very familiar about the way Turtle tells us this. She admits her own behavior pattern—going back to check—even though she thinks it's silly (she compares her behavior to checking for monsters). It's a down to earth way of sharing information, and she doesn't try to make herself seem tougher or otherwise different from how she really is. And the language she uses is conversational. There's nothing fancy going on here, just a kid telling her tale.