He is voiced by Tarusuke Shingaki in the Japanese version of the anime, and by Ricco Farjado in the English version of the anime.
Mirio normally comes across as an boisterous and very energetic boy. He has a very optimistic personality and usually has a smile in his face. His personality is what led Sir Nighteye to believe that he could be the one to replace All Might as the symbol of peace.
Mirio does not allow his laid back nature to cloud his judgment or impede his dreams of becoming a hero. He is a upstanding, humble and very strong boy, shown in his dedication to protecting others.
Like many others, Mirio was inspired by All Might. He is an self-sacrificing boy who intends to work for the greater good rather than any sort of personal gain. Knowing he can't rescue everyone, Mirio chose the hero name "Lemillion" because he aspires to save at least one million people. Despite Mirio's uplifting personality, he can suffer from failure just like everyone else. He has an excellent understanding of hero protocols and can control his emotions proficiently in order to see the bigger picture.