Alder is currently the champion of Unova. In the past, Alder's starter Pokémon died of illness, and in despair over not being able to save it, he began traveling around aimlessly. Before his partner's death, the two of them madly pursued strength... But the Pokémon's death changed Alder's outlook on life. 

His voice should convey his views on life; he should sound like he doesn't worry about the future, but mainly cares about enjoying the present. I imagine him having a low voice.