There's been a lot of attention geared toward My Hero Academia's spin-off, Vigilantes, lately as the latest arc of the series has been explored the mysterious past of Shota Aizawa before he became an official hero. This arc has been illuminating for several reasons because it's shown his time at U.A. Academy before he went solo, and revealed the identity of the even more mysterious Shirakumo that Aizawa didn't want to talk about in the main series. But as the arc came to its tragic end, it also revealed why Aizawa decided to be the underground hero fans know today.
Chapter 65 of the series sees Aizawa try and deal with the fact that one of his best friends has died, and following his death, Aizawa had thrown himself completely into his hero classes and decided to no longer rely on a team and instead venture out on his own.
The series confirmed that Shirakumo indeed was killed by the villain that attacked in the previous chapter, and Aizawa was lost as to what to pursue next. His teachers were worrying about him as he began phoning it in once he got passing marks in all of his classes. But he still throws himself completely into his hero classes, perhaps even more so in order to make up for the gaps left by Shirakumo's absence.
But while he was planning to open an agency alongside Shirakumo, Present Mic, and Midnight, after Shirakumo's death, Aizawa became a loner. He even tells his teacher that he'll focus purely on anti-villain combat, and will only give his all with fighting going forward. He vows to become an independent, underground hero because it's who he meant to be.
The death of his closest friend pushed him into isolation, and from then on Aizawa fought alone. Eventually getting the reputation fans know him for in the main series, Vigilantes has done more for Aizawa's past than the main series has. So now's the best time to catch up for sure.
My Hero Academia: Vigilantes is a spin-off series based on original concepts from Kohei Horikoshi's My Hero Academia. The series written by Hideyuki Furuhashi and illustrated by Betten Court and is based on original concepts by My Hero Academia series creator Kohei Horikoshi. You can currently read new chapters of the series at Viz Media for free at the link here, and the entire back catalog with a Shonen Jump subscription.
My Hero Academia: Vigilantes is officially described as such, "My Hero Academia: Vigilantes is set in a superpowered society, where there is nothing ordinary about evil anymore. Heroes, trained and licensed to protect and defend the public against supervillains, stand above all the rest. Not everyone can be a hero, however, and there are those who would use their powers to serve the people without legal sanction. But do they fight for justice in the shadows, or for reasons known only to themselves? Whatever they fight for, they are called... vigilantes."