Anybody could be in the Center. It depends on how the caller has set up the Promenade from the previous calls. You can even have 2 girls or 2 boys promenading together.

If you start from a position where you are not paired as couples, you would form a normal couple (Boy on left, Girl on right), and start promenading. In this case, a Reverse Promenade would have the Girl on the inside.