All Might's hero persona appears to be derived from comic book superhero stereotypes: colorful and filled with dramatic flair. Always sporting a giant smile, All Might shows a bright, borderline obnoxious, can-do attitude that is said to inspire others as well as help them feel safe and hopeful. He is extremely friendly and amicable due to his popularity and years in the spotlight, always taking his time to interact with fans.

When All Might reverts to his true form, however, he becomes less energetic and more serious, generally taking a milder stance so as to not attract much attention to himself, in direct contrast to the grandiosity of his hero identity. In this form, he frequently vomits blood as an indicator of his fragile health, although it might happen when he gets surprised or amused enough as well. Two personality traits that seem to be always present on him regardless of which form All Might is currently taking are his optimism and oafishness.

He has always had faith and hopes that someone would come along, worthy enough to bear One For All. Enough so, that he was willing to pass the power onto Izuku Midoriya soon after meeting the young man. All Might admitted that he had started losing faith in his ability to protect the world until Izuku's heroism taught him that something can always be done to fight injustice.

Izuku has compared All Might's personality to their mentor Gran Torino. Both apparently have two sides and are often playing dumb. All Might is actually more intelligent than his image would suggest, but he is still sometimes outsmarted by his students. He struggles as both a teacher of Class 1-A and mentor to Izuku. He has to use a script while teaching his class and often fails to help Izuku improve his mastery over One For All. His fellow teacher, Shota Aizawa calls him an idiot on occasion and Gran Torino says that all his strength and fame has not translated well into teaching others.

While All Might is seen smiling, laughing, and cheerful most of the time, he is extremely protective of his students and his rage will show if they're in danger. He was willing to use his Quirk beyond his time limit in order to save his students from the League of Villains. In spite of his anger, he made sure his students felt safe by showcasing his trademark smile before the fight.

All Might has a fatherly attitude towards his students (Izuku in particular) and takes pride in helping better his students as heroes. He is quick to give encouraging advice to them and often says he is proud of their improvements. On several occasions, All Might has tried to help mend Katsuki's pride and his relationship with Izuku. All Might was not afraid of pushing Izuku and Katsuki during their final exams and pulled no punches because he knew he needed to do his best to teach them both a lesson. He even used a villainous persona during their battle to get his point across, which both Izuku and Katsuki described as very intimidating.[3] He also has a habit of adding "young" (少年 Shonen?) to the surname of his students whether he is speaking out loud or thinking to himself (ex. "Young Midoriya").