For me, characters that rely on perfection, and fast execution timing seem the hardest to master for me. I played around with the different characters for a bit, and Karas seems the hardest to master. One mistake, and you lose a lot of health. His timing on his chain combo’s and rekka’s take more time to practice than most other combos.
I’m curious to see what other people’s opinions are of who the hardest character to master is.