Suppose that Haka has been incapacitated, uses his incapacitated ability to 'Select a hero. Decrease the next damage dealt to that hero by 2', and selects Mr. Fixer.
Suppose, then, that Mr. Fixer has Driving Mantis in play, which allows him to redirect damage that would be dealt to him if it is 2 or less, and then Mr. Fixer is attacked for 3.
My guess is that the way this would be resolved is that the attack would be reduced to 1, Mr. Fixer may redirect the attack, the amount of the attack remains reduced from 3 to 1, despite the fact that Mr. Fixer is no longer the target, and the next attack against Mr. Fixer would not be reduced.
I think another reasonable interpretation is that all of the above is correct except that the attack is reset to 3 for whatever the next target is. Another possibility, which I don't like as much, is that after the redirection, Mr. Fixer actually maintains the reduction of 2 for his next attack because he wasn't actually hit.
By definition, Haka's character card would have been in play before Driving Mantis, but what if this reduction had come from another card that came into play _AFTER_ Driving Mantis?
The only cards I can think of that redirect an attack based on the amount of the attack are Driving Mantis and Synaptic Interruption, and I can't think of any cards other than character cards that can reduce the next attack on Mr. Fixer or Tachyon. Haka has Haka of Shielding, but it only works on him.