Only being rude to his brother Piks, and being offhandedly nice to other people, his voice would change a lot towards others and Piks, with a much more threatening tone, and unnice things. (Not much on the pitch, keep it the same.) To pronounce that, It's basically like Piks, without his S.

"Hey scardy Pik, if your afraid,  you should still be a Pichu!"

"Morning, mum, I've made juice.." Quickly change tone to Piks'. "And your not getting much."