The main voice in When You Reach Me is Miranda's, and throughout the novel we learn all about the things going on in her head. Her most intimate thoughts and feelings are revealed to us as she tells us about the events that happened last fall. Miranda doesn't edit much out. We know when she's irritated with her mom (3.31-3.32), the kids at school who she really dislikes (11.9), and when she's really missing Sal (Chapter 38). Because she doesn't hold back, we're free to connect with the character.
Because of the mysterious elements in the novel, the tone can be also pretty darn creepy – especially when Miranda references that "you" who has asked her to write a letter (1.5). We've been known to get goosebumps each time she turns to address the unknown person.