So, I was playing earlier, and had 2 Despair to spend, but the attack against me was a miss, and therefore I didn't have to use Parry, and could not then use Improved Parry. Seems kind of odd to me that I can only riposte if I'm hit.
Another thing that came up was whether I could use Improved Parry when blocking an attack at someone else while using Circle of Shelter.
Finally, can I use Circle of Shelter to Parry an attack that the target (my ally) has already used Parry on himself?