I have not yet completed the main story, this could be missable, I will update once I have completed it.
After the 3rd Region (Kinki) is available, you have access to a Main Mission 'Spider Nest Castle'.
Completing this unlocks the Hidden Teahouse option in the Starting Point menu - and it's in here how I unlocked this Trophy.
Upto this point you may have fought Revenants and wondered what "Glory" was for that you obtained by defeating them - Glory is the currency you spend in the Teahouse.
To unlock the Trophy, you need to 'Transform' into another character - Hattori Hanzo is the cheapest option (and is a cool looking skin!), I bought him for 1300 Glory, equipped his character skin and the Trophy unlocks at this point.....