I have always found the difference between Kero and all other FPC 5 stars enough to be significant. If Kero is banned I drop down to one of the 4* cards. 8 Base power is key for overcoming attack manip and SOA cards.
@Falco: Makes sense, but I think that's because FPC have incredible 4* options as well. Back to the 5*s, though: Yoshida is fairly stable, especially if your deck is solid against SoA. Kuei is another good option that's a bit less stable yet solid, retaining Kero's base power and damage, and carrying a flexible but conditional SoB. Even Nekurenbo works as a sort of second-rate Kero, giving up that extra point of power manipulation for an extra point of life gap.
Look at some other perma-bans like Azel, Eddie, Spyke, Sylth Cr, etc. and you'll see what I mean: in the context of their clan, those cards are virtually irreplaceable. In some cases you have cards like Saki or Moses, far from irreplaceable, but it's evident how their removal breaks up inter-clan synergies which would otherwise be too good. Kero doesn't really fall into either of those categories.
I don't think she should be perma banned, just pointing out that I wouldn't use the other FPC 5* options. Several are solid, but that small drop off makes a big difference. On weeks where players ban Kero I still see FPC half decks, however almost all use 1-2 4 star cards instead of a 5 star.