King is an emotional and compassionate person, but displays behavior that would be perceived as a bit immature. (Spoiler) I expect whoever voices King to have a somewhat childish sounding voice, not unlike the English VA's. The voice actor must be able to sound like a  teenager/young adult, as he goes through a transformation during his battle with Mael. Preferably, the VA will be also be able to use a deeper, more strained voice. (He turns into an obese man in his human form.) If you're unable to do the deeper voices, I am happy to look for someone to voice them if I like your line delivery.