The only current option for your off-hand equipment slot is a shield. A shield allows you to create a defensive barrier, which can be maintained indefinitely without penalty but which prevents you from attacking, slows your movement, and degrades when struck.

My suggestion is to implement an alternative to a shield which resembles a parrying dagger. The intent is that it should be a more mobile, more aggressive, and less protective alternative, as follows:

• Instead of creating a barrier as long as "Defend" is held, a parrying dagger's barrier would only last a short time - maybe half a second - and would have a similar short cooldown after use.
• Like a shield, the barrier from a parrying dagger would degrade when struck, and eventually break and become temporarily unusable. However, a successful deflection would also trigger a counter-attack. The exact nature of this counter attack could vary between dagger families. Your basic dagger might just make a straight-forward strike at the attacker, but you could also have a broad-bladed knife which makes a sweeping counter attack that can hit multiple enemies, or a slim throwing dagger which can counter ranged attacks.
• A parrying dagger's barrier would not slow your movement, or would slow it less than a shield does. Some daggers could have a special effect when used while moving, allowing you to dodge like the enemies do.