Warning! Major spoilers for My Hero Academia: Vigilantes Chapter 64 below!
Although My Hero Academia's spin-off Vigilantes started out with the already interesting premise of taking place a few years before the start of the main series, the latest arc has definitely been a must read for fans as it has been sharing crucial parts of Aizawa's past as a student at U.A. Academy. Not only has this arc shed light on the reasons he wanted to be a hero in the first place, it also introduced the mysterious "Shirakumo" only mentioned in passing by Aizawa and Present Mic.
But there's been a huge cloud hanging over the arc as we were eventually going to figure out what happened to Oboro Shirakumo, and why Aizawa doesn't want to talk about him. By the looks of the end of Chapter 64, Shirakumo has died.
There was a pretty ominous cliffhanger heading into Chapter 64 of the series that saw Shirakumo unable to fight off a powerful villain. The chapter starts off proper with Aizawa in mid-fight with this villain, and when he starts to doubt himself, he begins to hear Shirakumo's voice through his communicator cheering him on. This gives him enough of a final push to defeat the villain once and for all, but the good times end almost immediately.
It's here that Aizawa finally sees the wreckage around him and gets an update on Shirakumo's status. He's happy Shirakumo cheered him on, but wants him to get treated for his wounds as soon as they can. But the others are clearly affected by something, and it's revealed that the speaker Aizawa heard Shirakumo through was broken. Aizawa believes he heard Shirakumo's voice, but he couldn't have really.
Soon the realization hits as Aizawa looks over the fallen rubble around him. With blood splattered all over, the worst is potentially possible as Shirakumo looks to have been killed. With a series like this, he could turn out alive in the next chapter but this also explains why Aizawa and Present Mic don't really want to talk about Shirakumo in the main series. As he's been tragically killed before he had a real chance at being a hero.
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."