Who is the the protagonist in "Raymond's Run"?

The protagonist is Squeaky, whose real name is Hazel Elizabeth Deborah Parker.

The word "protagonist" means the main character of the story, or the person the story is mostly about.  It is the hero of the story.

This story's protagonist, Squeaky, is a very interesting character.  She is strong-willed, but empathetic.  Squeaky has two passions in life—running, and her older brother Raymond.  Squeaky is very protective of Raymond and looks out for him.  Raymond has mental handicaps.  He is big, but has a younger child’s mind.  This often leads people to tease him, and Squeaky stops them.

In addition to taking care of her brother, Squeaky’s hobby is running.  It is more than just a casual pastime for her.  She is a very good runner, and she practices constantly so that she can get even better.  All Squeaky thinks about is beating the other kids in the May Day race.

There is no track meet that I don’t win the first-place medal. I used to win the twenty-yard dash when I was a little kid in kindergarten. Nowadays, it’s the fiftyyard dash. And tomorrow I’m subject to run the quarter-meter relay all by myself and come in first, second, and third.

Squeaky’s nickname is Mercury because she is so fast.  She trains for running by running often and practicing special moves that make her mother embarrassed to be around her.  For Squeaky, running is easier than being around other girls.  She just doesn’t quite get along with the neighborhood children and isn't close to any of them.  She thinks the girls are disingenuous. 

The race turns out to be the key to getting Squeaky to realize that other girls can be her friend.  She admires Gretchen, previously a sworn enemy, for coming in second. 

“In second place— Miss Gretchen P. Lewis.” And I look over at Gretchen wondering what the “P” stands for. And I smile. Cause she’s good, no doubt about it. Maybe she’d like to help me coach Raymond; she obviously is serious about running, as any fool can see.

Squeaky realizes that people other than her like running, and running can be a good common ground for her.  Since she watched her brother run a race of his own parallel to her, she sees running as a way to get closer to him too.   She and her brother finally have something in common, and it is the same thing she has in common with Gretchen.