Nappa is a big meathead. A lackey. A tough guy who follows orders. A brute who isn't an idiot but he's no genius either. There have been numerous voices for Nappa across the series, but all of the performances seem to follow these basic themes. Also, I plan to have Nappa go Oozaru at some point in the script, so the VA should be able to do a giant monster voice on top of the Nappa voice.