Popple, also referred to as the Shadow thief, is a thief from Bean Bean Kingdom. Along his travels he finds an Amnesiac Bowser, who he dubs "Rookie" and brainwashes into being his sidekick. He appears multiple times to fight the Mario brothers, and even gains Birdo as a new rookie to replace Bowser.

For his voice I would imagine Popple sounds similar to Will Stamper's cat voice (https://youtu.be/nVA7pOrw3bo?t=14) So try to be as close to that as you can!