He may have simply been the first to volunteer for the experiments in order to gain power or recognition. There's really no telling what he was doing beforehand. He could have simply been a citizen of Vector, or maybe a high ranking official. We just cant know.

I, personally, would think he was a random, patriotic volunteer, possibly a low ranking soldier. Mainly because I dont see the Empire wanting to risk one of their important people on a process that could easily backfire (and did). I see Kefka as having risen to favor with the Emporer after this, not before.

But again, that's all just speculation.

And I fully agree with all the assumptions of ambassador.