Of the teachers at My Hero Academia's U.A. Academy, Shota Aizawa may just be the most popular. His sardonic sense of humor and gloomy demeanor have helped him to garner a sizable fanbase, and led to his character playing a major role in the spin-off prequel series, My Hero Academia Vigilantes. The end result, however, is that a major part of his past is almost entirely missing from the main series manga.
While the name "Oboro Shirakumo" had meaning to Aizawa, readers of the main manga were rightly confused when it came up. He had a brief cameo appearance in chapter 216, and in chapters 253 to 255, Aizawa and Hizashi Yamada (better known by his hero name, Present Mic) speak obliquely about Shirakumo and their time together, trying to get information out of the villainous Kurogiri by reminding him of his past, but only one word comes of it: "hospital."
The full story of Shirakumo is only revealed in Vigilantes, chapters 59-65. Aizawa recounts (via extended flashback) his days as a student at U.A., including the work-study program where he worked with Shirakumo, whose hero name was "Loud Cloud," and Midnight at the Purple Revolution Agency. Aizawa, Shirakumo, and Yamada were close friends at school, and planned to open a hero agency together. However, when the Purple Revolution Agency, led by His Purple Highness, responded to a villain attack, Shirakumo was tragically killed by falling debris, cutting short his hero career and turning the already anxious and gloomy Aizawa onto an even more cynical path. In main series chapters 253-255, it's said that All for One stole Shirakumo's body after this and used it as the base for the Nomu that would come to be known as Kurogiri.
While the bare minimum is shown in My Hero Academia, the arc in Vigilantes reveals much more of Shirakumo's character, personality, and the relationship that he and Aizawa had, as well as how this tragedy shaped Aizawa's career as a hero (even Aizawa's iconic goggles were a gift from Shirakumo). Much of the reason he seems so hard on his students, particularly in the context of their hero work-study, is because he knows that even student heroes can die in the line of duty. Shirakumo is said to have been designed as a foil for Aizawa, the upbeat Mirio-like student to Aizawa's Midoriya-style overthinking. His name, Shirakumo, means "White Cloud," reflecting his cloud-like hair and cloud creation quirk, and is mirrored by Kurogiri's name, which means "Black Fog." The reveal of this duality is clearly meant to be a big deal, but without the context of Vigilantes, it lacks the intended punch.
Fans of Aizawa who have skipped My Hero Academia Vigilantes would do well to go ahead and read the six flashback chapters dealing with Shirakumo, which offer a new angle on a fan favorite, helping to cast Aizawa's gloomy disposition and high expectations of his students in a new light, as well as adding to the tragedy surrounding Kurogiri's origins.