Defensive Dueling

When you are wielding a finesse weapon with which you are proficient and another creature hits you with a melee attack, you can use your reaction to add your proficiency bonus to your AC for that attack, potentially causing the attack to miss you.

In Addition:
Daggers gain the "Offhand" tag, allowing them to be wielded without penalty alongside non-Light tagged weapons. 
You can still apply AC bonus with a non-finesse or non-light weapon so long as you wield a dagger in your offhand.

Ex: Rapier & Dagger fencing.