Age: 18 (first appearance)
Birthday: July 15
Height: 181 cm (5'11¼")
Quirk: Permeation (ability to phase own body through physical matter)
He is third-year student at U.A. High School and is part of The Big 3 (the strongest three hero students in all of Japan). Before Izuku Midoriya, Mirio was the top candidate for All Might's successor and inheritor of One For All.
During the Shie Hassaikai Arc Mirio risked his life and sacrificed his Quirk to atone for putting Eri in danger. He even continued fighting and protected Eri from Overhaul using his physical abilities. Despite losing his Quirk and his mentor following the raid, Mirio remains steadfast in his hopes to become a hero. He refused Izuku's offer to receive One For All and told his friend Tamaki he was in no rush to get his powers back.
(Source: My Hero Academia Wikia)