What to do with +HP -Res Karel

He is going to have 50 HP so you could go with Fury, Desperation, Panic ploy (C or S) and a threaten / buff skill if you want to use the seal Panic seal. He won't be too bad I don't think.

Honestly I'd keep his base kit. Give him your choice of C skill and support as well as brash assault and he'll shine sub 50% At +hp keeping Reprisal over Moonbow can even be good for the solid damage boost it gives him when he gets really low.