Courtney is a good friend to Marcus.  Since she is also from an army family, she understands what Marcus is going through.   She is there to support him.  Even though he cannot spend as much time with her as he would like because he is taking care of his sister, she gets it.

Marcus explains how he feels about living on base.

There was just something about everybody having something important in common, everybody understanding what you were thinking and what you were going through. (ch 1)

When Marcus is called out of class, the other kids tease him and think he is in trouble.  They are not from military families.  Courtney empathizes with Marcus, realizing that he is worried that he is being called out of class because something has happened to his father.  Courtney’s father ends up being killed, and Marcus feels bad because his returned.