In Mortal Kombat 11, there is an unlockable character called Frost. She is the only character that needs to be manually unlocked. All others are unlocked from the beginning.
How do I unlock Frost? Play Chapter 4 in Story Mode and always choose Sub-Zero (a total of 4 choices in this chapter). If you do that, you will freeze frost after the chapter!
Have you seen the Mortal Kombat 11 Trophies & Achievements? Here you can find all PS4 trophies, including tips and solutions.