Riolu, the cocky, arrogant leader of Team Aura. Not what you'd exactly expect from the main protagonist. As a human, he was a very successfull Pokemon Trainer, and has an extensive knowledge of most Pokemon's strengths, weaknesses, types, etc.
In the begginning, he's not very good at battling besides giving orders to his team. He's a master battle tactician, able to react in battle quickly, use the environment to his advantage, and everything else that makes a good leader/fighter.
Only problem being his transformation into a Pokemon wasn't fully complete (it'll be explained better in the story), so in the beginning he has no access to any moves...
He uses his new body to his advantage, using a Pokemon's innate ability for battle to train harder than any human ever could! One of the few things he likes in the entire pokemon world he's in is the fighting, and the rush he gets after each battle.
He is very judgemental of everyone around him, especially the pokemon (since they aren't human). On his first night in the pokemon world, he tosses away his "dumb birds next of a hay bed" and goes looking for a real bed. With a mattress and everything!
He may lash out at anyone who disrespects him, his pride or his humanity. But deep down he's just a scared, confused young adult trying to get home, having to fight every day just to survive in a body that's not even his. In a world thats not even his! Deep down he's very afraid, and is super happy with any progress he makes on getting home. Although he'll never admit, he eventually begins to care for his partners.
For his voice, go for something cocky, proud of themselves. With a bit of gruff in the back of their voice. In human years they're around 18-22 so try and emulate that. American accent is preffered but if you aren't American you can still audition lol.
(For examples of his voice/personality. Think of Paul from the Diamond and Pearl anime, or Vegeta from DBZ)