Kenny is the nerdy little turd who was in the construction yard that you rescued and taught how to craft weapons early on in the game (if you did that side-quest). Once you begin Chapter 7, you will get a notification that Kenny has kidnapped someone. While he has a lot of health, he is easily stunned by melee attacks. If you combo a weapon during combat, this will also distract him. After the fight, the end decision is yours to make and the achievement will come either way.