Who is Miranda from When You Reach Me and what is their importance?

Miranda is a sixth grade student living in New York City during the late 1970s. Miranda is the only child of a single mother and has had the same best friend since she was a baby. Now, however, Miranda discovers that her best friend no longer wants to be her friend for reasons he has not explained to her. This forces Miranda to make friends with girls her own age at school so that she is not forced to be alone.