Enemy Aggression. Enemies will attack party members causing the most damage or that are using specific Art types. This raises the aggression of the enemy causing them to switch their target. There are certain arts, skills, and gems that lower aggression for party members not equipped for taking damage, or raise aggression for party members equipped to take the damage. Handling enemy aggression in a battle can mean the difference between defeat and victory.
In the image, the red ring indicates that enemy is targeting Reyn.