Mirio normally comes across as extremely goofy and very energetic with some peculiar habits. Going solely based on his personality, many tend to question how Mirio is U.A.'s top student. Mirio has a very optimistic attitude and is also almost always smiling. His personality is what lead Sir Nighteye to believe that he could be the man to replace All Might as the symbol of peace.[2]

Mirio does not allow his laid back nature to cloud his judgment or impede his dreams of becoming a hero. He has proven to be an extremely hard worker, turning a near unmasterable ability into one of the world's most powerful Quirks. Mirio is constantly looking for ways to improve his skills and tries to work toward his goals by accumulating different experiences to learn from them.[3]

Like many young men and women, Mirio was inspired by All Might to become a hero. Knowing he can't rescue everyone, Mirio chose the hero name "Lemillion" because he aspires to be a hero who can save over 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 controlling his emotions in order to see the bigger picture.[4]

Lemillion and Deku acted for the greater good when they chose to allow Eri to leave with Overhaul. However, their failure to rescue her tormented them once they found out the truth. This put a large damper on Mirio's normally cheerful personality and gave him immense guilt. His guilt drove him to promise to never let Eri come to harm again, a promise he would keep no matter what. [5]

Mirio is also extremely brave and in contrast to his laidback nature, he is experienced in dealing with dangerous people and does not scare easily. He was able to talk to Overhaul in his normal, cheerful manner despite knowing Overhaul is a dangerous Yakuza criminal.[4] He also went ahead of the entire combined hero force during the Shie Hassaikai Raid and combated Overhaul, Chronostasis, Deidoro Sakaki, and Shin Nemoto all by himself.[6]

Alongside his bravery, Mirio has shown to be confident in his abilities. He knew he could take on all of Class 1-A to teach them a lesson and was even bold enough to face four of the Shie Hassaikai alone. Mirio clearly believes in his strength and always shouts "POWERR!" after winning a fight.[3][6]

Lemillion has shown the values of a true hero. He risked his life and even sacrificed his Quirk to atone for putting Eri in danger. Despite knowing what it would do to his career, Mirio allowed his Quirk to be destroyed in order to inspire Eri and keep his promise to never let her know pain again. It was Mirio's bravery that allowed Eri to awaken and set the path for the heroes to defeat the Yakuza. Lemillion even continued fighting after losing his Quirk and protected Eri from Overhaul. His incredible tenacity caused Shin Nemoto to question whether he was even human.[7]

Despite losing his Quirk and his mentor following the raid, Mirio remains steadfast in his hopes to become a hero. His cheerful personality has not faded and he is empowered by Sir Nighteye's final words to him. He even refused Deku's offer to receive One For All[8] and told Tamaki he was in no rush to get his powers back.[9]