My Hero Academia's full of plenty of mysteries, and one of the more intriguing was the secret between Aizawa and Present Mic as the two once referenced a mysterious "Shirakumo" before Aizawa refused to bring up this person again. But while the main series explored other mysteries, the My Hero Academia: Vigilantes spin-off actually just rounded out an entire arc focusing on Aizawa and Present Mic's time at U.A. Academy before becoming pro heroes. It's here that fans were truly introduced to this Shirakumo, who turned out to be one of their closest friends.
My Hero Academia: Vigilantes has had some smaller ties into the main series, but My Hero Academia Chapter 253 provided the most direct tie into this spin-off yet as it teases that Kurogiri is actually a created being based off of their old friend Oboro Shirakumo.
My Hero Academia: Vigilantes takes place five years before the events of the main series, so there have been a few cameos of pros and other characters here and there but the Aizawa arc changed things. Its story about Oboro Shirakumo's friendship with Aizawa, and unfortunate death, has now been completely confirmed to be part of the main series' canon rather than just a story that could exist on its own.
There's a brief look at Shirakumo at the end of Chapter 253 also confirms that his design is completely in line with the main series too. While it's to be expected that the spin-off series would have some ties into the main series, it's another thing to have a completely crucial part of the narrative tucked away into Vigilantes.
It's possible that the main series could explore Aizawa and Present Mic's past with Shirakumo more, but Vigilantes already took the time out to not only flesh out their friendship but reveal just how crucial Shirakumo was to Aizawa's self-confidence. In fact, we learn in the spin-off that the loss of his friend is what drove Aizawa to become an underground pro-hero.
With this direct connection, this now makes Vigilantes a much-needed read for those who haven't checked out the spin-off just yet. Written by Hideyuki Furuhashi with illustrations provided by Betten Court, and based on original concepts from My Hero Academia series creator Kohei Horikoshi, you can currently read new chapters of My Hero Academia: Vigilantes 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."