Hmm was it mentioned, explicitly or hinted at during the show? Cause I can't recall if he ever mention that being a king would give him the most power to do good deeds for people. If that were to be made clear, then that would give his motive some backbone to it. Or was it mentioned in the .5 episodes that I haven't seen any of?
I agree that a huge reason why Sougo didn't work was his lack of charisma. I just can't see him as someone that can carry the show or plot forward. His naivety is played downright to childlike, and unless something dark/major happened during post-Christmas episodes that will lead him to suddenly "mature out" (cue 2nd half of Kiva) I can't see how we are supposed to be invested in him as a character.