You can earn this trophy once you have the Hidden Teahouse unlocked.
To unlock the Hidden Teahouse you need to complete the Spider Nest Castle level in Kinki.
Once unlocked you can trade the Glory you have earnt from defeating Revenants for a Girl skin you can transform into. You can also earn glory from playing Co-Op online.
The cheapest female skin is 70.000 glory, so you will need to defeat around 300-400 high level revenants to get this, depending on how much glory they drop!