Kohei Horikoshi has dropped several peculiar nuggets and teases over the course of My Hero Academia's five year run, but one of the most mysterious was the "Shirakumo" that Aizawa and Present Mic briefly brought up in one chapter before Aizawa refused to mention the name ever again. But surprisingly, the spin-off My Hero Academia: Vigilantes introduced the mysterious Oboro Shirakumo in full when it explored Aizawa's past at U.A. Academy before becoming an official pro hero. It's through this spin-off that fans also found out why Aizawa refuses to talk about Shirakumo and why he's not in the main series.
But this changed in the latest chapter of the main series as it featured its most direct tie-in to the Vigilantes spin-off to date, and unfortunately brought "Shirakumo" into the main series years after his death. While this is a reunion, it's far more tragic as Shirakumo's corpse has been used as the foundation for a new kind of Nomu.
Chapter 254 of the series introduced Oboro Shirakumo to fans who most likely have not read through the Aizawa arc of My Hero Academia: Vigilantes. Confirming that he indeed died during a work study in a battle against a terrible villain, it also explains that Shirakumo played a crucial role in Aizawa wanting to become a pro hero in the first place. Because he went to U.A., but he was disconnected with the school and his studies.
But Shirakumo had inspired him, and they were planning to start a hero agency of their own together with Present Mic and Midnight. Shirakumo was Aizawa's best friend, but was killed by falling rubble when a monstrous villain attacked he and the pro hero their were studying under, His Purple Highness. Shirakumo's death put Aizawa down his path of becoming an underground hero, and now that Shirakumo's essence seemingly lives on through Kurogiri, Aizawa is making a desperate plea that he might be somewhere in there.
My Hero Academia was created by Kohei Horikoshi and has been running in Shueisha's Weekly Shonen Jump since July 2014. The story follows Izuku Midoriya, who lives in a world where everyone has powers, even though he was born without them. Dreaming to become a superhero anyway, he's eventually scouted by the world's best hero All Might and enrolls in a school for professional heroes. The series has been licensed by Viz Media for an English language release since 2015. My Hero Academia will also be launching its second big movie, Heroes Rising, in Japan this month.