Kemper and an organ

Yes i have this.

I'm afraid it won't work well since the KPA is a computer made to treat some guitar signals in a particular frequency range.

I did some tests with a keyboard and an AKS style synth on my KPA, even with AMP and CAB off, and it clearly lacked some frequencies.

it's ok in your case since JC120 is still a guitar amp but on a FR system ...

Maybe try tube-preamp-only Profiles with the gain dialled in sufficiently to ad a bit o' grit?

Yes - it works fantastically. Combo organs, Rhodes, Wurly, Clav are all traditionally (and best) performed and recorded through tube amps, and in fact Jon Lord famously used a Marshall with his Hammond.
The limited frequencies mentioned earlier are part of what is good about it, but you can also use direct profiles with no cab sims if you choose. You would not use it with a super clean synth pad, strings that you do not want to be grainy.
The one thing to watch is the input volume - most keys are hotter than a guitar input.
But yes, it works EXTREMELY well and that is the reason I own a Kemper as I do not (really) play guitar.

Yes - it works fantastic. Combo organs, Rhodes, Wurly, Clav are all traditionally (and best) performed and recorded through tube amps, and in fact Jon Lord famously used a Marshall with his Hammond.
The limited frequencies mentioned earlier are part of what is good about it, but you can also use direct profiles with no cab sims if you choose. You would not use it with a super clean synth pad, strings that you do not want to be grainy.
The one thing to watch is the input volume - most keys are hotter than a guitar input.
But yes, it works EXTREMELY well and that is the reason I own a Kemper as I do not (really) play guitar.

Oh, settings - on electric pianos, start with some clean Fender profiles, and even dial the gain back a bit from there. Turn up the pick articulation in the amp settings. This allows the not attacks to poke through nicely.
On organs - depends on what you are going for. Combo organs sound great through AC30's and the like - again - dial the gain back to account for the hotter input, or if you are going to use the amp mostly for keys, you can globally change the input gain allowing you to step through presets to check things out.
Ventilator pedal in the external effects loop, after the amp section, with it's drive off is my go to "fake Leslie" when I can't bring the real deal. I will be profiling the Amp from one of my 147's one of these days to use in that application. I had a friend make me the requisite chord, all I need is a DI that can handle speaker level input and a few spare hours.

Hi, I'm not sure if this counts, but I put a EHX C3 pedal in my fx loop, before the amp, and then through a distorted Marshall.
As PhilAiken wrote: "Jon Lord famously used a Marshall with his Hammond".
It's sounds pretty convincing in my ears.