Does Mercy wear Arya’s face or someone else’s?

Mercedene (or Mercy) is a face that the House of Black and White use as part of their training process:

They brought a robe for her as well, the soft thick robe of an acolyte, black upon one side and white upon the other. “Wear this when you are here,” the priest said, “but know that you shall have little need of it for the present. On the morrow you will go to Izembaro to begin your first apprenticeship. Take what clothes you will from the vaults below. The city watch is looking for a certain ugly girl, known to frequent the Purple Harbor, so best you have a new face as well.” He cupped her chin, turned her head this way and that, nodded. “A pretty one this time, I think.”

The idea being that Mercy, as an established junior mummer, can go through a course of training in the art of method acting, which is necessary if a Faceless Man is to successfully pass themselves off as someone else, while not being important enough to raise suspicion.