To unlock this achievement, you'll have to change the gender of your character from male to female or vice versa. To do this, all you'll need to do is select 'Persona' from the main menu, then 'Character' and finally 'Gender'. While in gender, just choose the opposite gender to what your character is and you get the achievement.
EDIT 05/07/2017 - Thanks to SPACE ODDITY for the following information: If just changing your gender doesn't work, then change a Head feature on your character after changing the gender and this should unlock the achievement.