Kengo Legend of the 9

To obtain "Stealth Attack," most of your kills through one of the character stories have to be "instant kills" (I think it's something like 40% of your total kills), where you throw an enemy into an object such as a log or a wall while grappling (press X to grapple, and X again in a direction to throw them), and attack them when an orange arrow is above their head. Note that their stamina bar (green bar) has to be completely empty in order to do an insta-kill.

You can drag your enemies (Press X to lock swords then just move back and forth) to drain their stamina faster as well, but they may break the lock if you aren't careful.

This will unlock after you defeat the final boss and the criteria is met. Also note that you can only get one of "Stealth Attack," "Late Attack," and "Early Attack" in a run. You cannot get more than one of these per playthrough.