Our time travelling guide in this book is a sixth grader named Miranda. She speaks in the first person and often addresses an unnamed person ("you") in the narrative, who we later figure out is Marcus (the Laughing Man). Since she's the main character narrating her own story, we say that she's a "central" narrator.
The first-person narration gives us access to Miranda's observations and thought processes, both of which are crucial in this novel. The story offers so many mysteries, puzzles, and riddles, that Miranda's consciousness is the lifeline that pulls us through. Miranda isn't a genius – she's an average sixth grade girl. This means that she explains all of her thoughts very carefully, and it often takes her quite some time to put two and two together. This can be a real help for younger readers. See, for example, her narration of the day the Laughing Man saved Sal's life in Chapter 45.
Because we see her inner life, Miranda also becomes very real to us. She tells us about her hopes, her fears, and her desires. For examples, check out the way she describes her fascination with smells (24.5), or her struggles to get over fights with her friends (Chapter 38).