His j5C has strong priority when it's in motion. The only time you can counter it with a 2C is when he wiffs or is in the starting frames. It's normally better to barrier block this if you are unsure a 6D or a 2D will connect.
Haku has a pretty big hit box, so 6Ds will hit him in a lot of cases even when he's on the ground hopping at you.
When I fight Haku's I find it the most important to make sure he can't get near me. So, I tend to bait him into air dashing by running away a lot, and when I get a jD D > 214D in, I'll put down a gravity field to prevent him from moving for the moment.
As far as frames go, foward rolls are very unsafe unless you have a Dragon Punch (Ragna, Jin, Litchi, etc) If Haku rolls forward, if in range, you should be able to easily punish it with a 2B > 3C > gravity field etc combo, or a 236D > rapid > etc. Since this is actually hard to punish in many cases, I don't recommend doing it unless you are absolutely sure he will roll forward.