This is unlocked during chapter Pressure. In order to get there Fliss and Brad need to recognize each other. To do that, Brad has to stay undetected on Duke of Milan and needs to find a gaz mask by opening a locker with a jumpscare two times. Fliss has to snap out of it in the ball room by breathing clean air from the door. If all done correctly, Fliss and Brad won't see each other as demons.
After that both of them will move to the chapter Pressure. At some point during this chapter Fliss and Brad will be deciding who goes down the hole in the floor. In order to unlock the achievement as Fliss choose the following
WORRIED (you go)
FRIGHTENED (I'm not doing it)
That popped and achievement for me