Alacrity

You've mastered the art of moving through tight spaces.  As a bonus action, you can move anywhere in tight quarters, from the front of the battle line to the back or side, without provoking acts of opportunity.  This means that you can perform actions in battle such as a) moving out of the way of an attack, b) moving yourself into the way of an attack meant to hit your party member, or c) roll a DC check vs the incoming attack roll to make it miss entirely.