Your Mercenary’s Skills depend on their Designation. They provide boosts to certain Unit statistics under certain conditions, or Resource production and storage in your Base. There are 5 levels for each Skill. You can improve a Mercenary's Skill by using a Skill Core in the Mercenary Facility (“Collection” tab, then “Skills” tab). Note: You can choose the Mercenary when you apply a Skill Core, but the Skill leveled up will be chosen at random.