The thing that irks me is that people believe Karok will replace Fiona.
Karok absorbs attacks and powers through them. If this video is any example, there is a large chance that he could be taking heavy damage if played to be offensive. Karok is a wrestler type, to stop the boss at certain times, and inflict damage all the time.
Fiona is a dancer, blocking, guarding, and flying back until the perfect moment is open for a normal attack. Her stun and large amounts of damage while the monster attacks is what separates her from a Lann right now.
Here's how I see it:
What separates Fiona from Lann is that Fiona will make her own opportunities to attack, whether it be with counterattack, shield bash or with her high knockdown, as opposed to Lann (and Evie's for that matter) method of attacking at only when the boss is open.
Fiona right now is the only character that can attack and defend at the same time (counterattack). Karok, however, is also able to attack and defend at the same time (damage absorption and boss grappling). The difference here is that not only can Karok heal while doing damage, he can also regularly and effectively attack against a boss's smash attack with no penalty (in fact he comes out on the plus side when attacking), while Fiona's Heavy Stander Counterattack supposedly sucks from what I hear.
Which also highlights another problem: a Fiona player must be able to discern between regular attacks and smash attacks. No other class needs to do this; it doesn't matter for any of them. However, if a Fiona player Guards a smash attack, they're down. If they heavy stand a regular attack, that was a wasted opportunity (Blood Lord has some annoying mixups, for instance). And this won't change when Karok comes in, because whether it be normal attack or smash, he can still power through it with damage absorption, so the only thing he needs to worry about is timing (which Fiona also needs to worry about if she wants to get a perfect Guard off).
Though in the end I think it'll all hinge off of how big of a window Karok has with his damage absorption skill. If it's as big as Fiona's perfect Guard window, then Fiona will become obsolete (perfect Guarding is pretty easy). If it's smaller, Fiona will have a chance.