In order to unlock this, you have to spare the life of Yang at the end of Mission 5 “The Sanctuary”.
To spare his life
DO NOT kill him in his second phase with a takedown, instead wait a few seconds until the prompt disappears and break his Structure again and you will see the option to spare his life with
. A third phase will start now and you have to a takedown again here to finish him. Don’t worry you can not die here.
IMPORTANT: You need to spare the life of all other bosses before you can spare Yang’s life. If you don’t do this, the spare option won’t appear.Tip: Blocking and performing attack fill the bar to break your opponents Structure. After skipping the opportunity to perform a takedown during the second stage of the fight, it may be safer to block attacks to fill your opponents structure bar than attacking, however if you monitor their health you should be fine to throw a few attacks in.