Who is the note writer in book when you reach me?

The note writer in the book "When You Reach Me" is Miranda, the main character.

Throughout the story, Miranda receives mysterious notes that seem to predict future events and offer cryptic advice. These notes, written in a distinctive style, are ultimately revealed to be written by Miranda herself, from her future self. This is part of the book's central mystery and themes of time travel, fate, and self-discovery.