Now more than ever, My Hero Academia is exploring just how imperfect and overly idealistic its hero world truly was. While the idea of a world full of superheroes sounds nice, the perpetuation of a black-and-white society may have inadvertently created new and greater monsters, as well as a policing body/class system that pressured more citizens than inspired them.
Midoriya Izuku has come a long way to overcome the limitations set on him at birth; but, despite his growth, he's found more and more flawed and tortured people along the way on both sides of the moral road. Like Deku, the story has evolved past one just becoming strong enough to become a hero, but also brave and kind enough to grow past one's anger, cowardice, and past.
Minoru Mineta
While many question how in the world Mineta ever got into U.A., there's no denying that he's currently giving the school his all. As one of the smaller kids with a more niche Quirk, Mineta has been far away from the front lines of action and can often be found either crying or complaining.
He still does plenty of that today, and Mineta got to prove during the Midterm Exam Arc that he is able to persevere and show some ingenuity. With a knocked-out exam partner and Midnight backing him against the wall, Mineta managed to formulate a plan that would both trap his instructor, but also allow him to haul his partner to the finish line.
Stain
To be fair, Stain hasn't exactly apologized for killing or injuring the heroes that he has. In all honestly, if he did manage to escape during the most recent arc, he has also likely returned to hunting down pro heroes. Despite this, some part of his heart had to have been melted during his interaction with Midoriya and co.
After seeing Deku tirelessly work to save his friends, Stain's trust in heroes and hope in humanity had somewhat been restored. At the very least, this interaction allowed him to save Deku from one of the Nomus, sparking the first time in a while that he actually protected a hero instead of attacked them.
Gentle Criminal
Gentle Criminal is a prime example of societal pressures and the glorification of heroes creating delusion and obsession. Gentle once aspired to become a hero himself, but he struggled and was eventually outright told to quit.
Demoralized by society's rejections and self-loathing, Danjuro Tobita would eventually create his Gentle Criminal persona and dedicated the rest of his life to petty crimes which he would film, the only way that he saw at the time to achieve some glory. After Deku defeated him, however, he took an opportunity to act like a hero again and deflected the authorities from arresting his partner, La Brava.
Momo Yaoyorozu
When Momo Yaoyorozu first debuted, she was primed to be one of the story's best female characters. She was cool, competent, and had one of the best Quirks in the series. She, however, wasn't able to practically prove herself, and, within the shadow of Todoroki and Deku, she lost her confidence and decisiveness as a hero.
Once one of the proudest characters in Class 1-A, Momo became just another insecure high school girl. The Midterm Exams, though, had the magic effect of lighting the fire under plenty of students, including Momo. With Todoroki's trust and Aizawa pushing her, Momo was able to show her smarts and find her confidence again.
Eijiro Kirishima
Eijiro Kirishima is the very embodiment of strength and endurance in this series. However, he didn't always have the loud, abrasive demeanor that he has nowadays. When he was younger, he was much more timid and insecure.
Despite having been inspired by Crimson Riot, Kirishima often struggled to fight back against bullies and even outright froze in fear when faced with a regular villain. His enrollment into U.A. was the first part of his journey to become the tough, enduring hero that he always wanted to be. The second came during his and Fat Gum's epic fight against the Shie Hassaikai.
Hawks
While Hawks himself hasn't come out as explicitly flawed, he strived to improve upon a very troubled childhood and family history. Keigo Takami is the son of a petty criminal who barricaded himself in their home to hide from a murder charge. After his father was eventually arrested, he and his mother continued to live in poverty.
Knowing nothing else than the teachings of his favorite hero, Hawks at a young age turned to saving people, eventually getting discovered by Public Safety. This would launch his hero career today, helping to improve his and his mother's situation while also hopefully making up for his own father's mistakes.
Twice
Twice is perhaps the only member of the original League of Villains whose growth during the Meta Liberation Arc didn't involve just committing more acts of violence. Arguably, his change was more positive; haunted by the existential terror of his Quirk, Twice always lived in doubt on whether or not he was the original body or just a copy.
This doubt would make him fearful and hesitant, unable to use his Quirk to its fullest. However, having found solace within the League, Twice managed to find himself again as well as the strength to fully use his Quirk.
Katsuki Bakugo
A lot of fans early on were calling out Bakugo for potentially turning to the dark side, and who could blame them? The guy was constantly angry, egotistical, and power-hungry. He didn't start out well in the series as the bully to the protagonist, and he didn't make a great case with each of the times he told Deku to die, as well as a few of the times that he tried to legitimately hurt him.
Though long and still ongoing today, Bakugo has managed to calm down across the story. While there's a couple of instances in the story of him begrudgingly helping Deku, the war arc had him actually sacrifice himself to save the kid he once bullied so harshly.
Shoto Todoroki
While Bakugo seemed like the series' Sasuke Uchiha because of his constant antagonist vibes, Shoto Todoroki arguably gave off more warning signs, as the edgy kid sinking into his own pit of despair. Having been bred and raised within a cold, abusive environment, Todoroki's only drive to become a hero was based on spite.
Defined only by his hate for his father, Todoroki refused to fully use his Quirk and dismissed the rest of his class as simple competition. His time with Deku has melted his heart in both a metaphorical and likely literal sense and has allowed him to see past his tumultuous family history to find his own path as well as forgiveness.
Endeavor
No other character in My Hero Academia has had a redemption arc as long, detailed, or warranted as Endeavor's. Having tortured his family for his own selfish grabs at greatness, Endeavor debuted as one of the series' most despised and seemingly irredeemable characters. After becoming the #1 Hero, Endeavor has strived to make things right with his family, including everything from apologies to supporting them from a comfortable distance.
Even now, he's faced with accepting his sins publicly and taking on his greatest failure and sin yet, Toya Todoroki. While it's controversial to say whether or not Endeavor has earned any redemption, there's no denying that he's working to be the man he always should've been.