I may be wrong, but I think dread fighters are immune to Shadow Sword backfires. Once again, I could be wrong(I haven't fact checked this), but I think that's why Zeke never backfired.
They’re not, they have a reduced backfire chance though, which is why it happens far less often with them, but it still can (I’ve had saber die to one way back when I first played the game) they are totally immune to them in gaiden however
