I had a similar essay question..
I definitely would focus on something not covered in your PS, primary app, or other parts of the secondary app. Pick something unique about you and use it to describe parts of you. Pick an experience, aspect of your life, something you value, etc. and work off that.
For mine I talked a little bit about how my family was important to me. This transitioned into how I came to learn about caring for other people, building relationships and that transitioned into why I wanted to be a doctor.
good luck