The Alpha Pokemon himself- Arceus! My take on him is that he is NOT a benevolent, smiling god like he initially is in the anime, rather he's a stoic-speaking neutral party that does as he feels when he recognizes something must be done, like bringing Dawn into the past to save Hisui. I imagine him to have a low, booming, attention-calling voice potentially with some manner of echoing effect? But I'm open to however you play him! He also speaks exclusively in Old English so beware.