Display More"Constant latency" is not supposed to fix the delay (your signal is sent through digital processing, which will always come with some latency) but it does make it easier to fix it.
Without it, the Kemper tries to minimize latency, which is good for live playing, but also means that latency depends on the effects used, and thus differs from rig to rig. With "Constant latency" on, the delay is always the same (I seem to remember it was 4.9 milliseconds), regardless of the current effects settings. That way, once you've figured out the correct offset, you can set it and forget it.
If you want to be very precise, use "Constant latency" and calculate the offset as follows (please correct me if I'm wrong here):
For instance, at 44.1 kHz:
Of course, your audio interface may introduce a bit of extra latency. I don't know Reaper very well, but many DAWs have a way to figure this out and compensate for it across the board.
In any case, once you've figured out a good offset value, it should remain the same as long as you stick with "Constant latency".
Thanks - this was my current understanding exactly.