WARNING: The following contains spoilers for My Hero Academia Season 5, Episode 19 “More Of A Hero Than Anyone,” now streaming on Funimation, Crunchyroll and Hulu.
Since its first season, the Nomu have been one of the biggest threats in My Hero Academia. The Quirk monsters have been some of the League of Villain’s greatest assets, shown to be capable of going head to head with top heroes like All Might and Endeavor. Even the weaker ones prove to be a handful for regular law enforcement and lower-level heroes. Their origins have been a mystery for years but Episode 19 of Season 5 finally puts all speculation to rest.
Experiments on the Nomu All Might defeated at USJ revealed that they were originally humans. Their unnatural appearance was just a result of their bodies being bio-engineered so they could handle multiple Quirks forced upon them. For a while, that was all anyone knew about the Nomu. But the latest information on them comes from a character nobody could have guessed was a Nomu himself: Kurogiri.
Regular Nomu are nothing more than puppets with no consciousness or will of their own but High Ends seem to be the exception to this rule. The Nomu that attacked Endeavor at Kyushu at the end of Season 4 of My Hero Academia showed clear signs of having its own personality but Kurogiri is on a whole other level. He's capable of intelligent conversation and can make really good tactical decisions. He also takes his role as Tomura Shigaraki's protector very seriously, even serving as an advisor of sorts to the young head of the League of Villains. He's capable of rational thought and isn't solely driven by destructive impulses. Even among the High Ends, Kurogiri is a cut above.
As it turns out, Kurogiri's drive to protect Shigaraki didn't emerge from a vacuum. Rather, Kurogiri is based on the personality of the man All For One corrupted to create him: Oboro Shirakumo. Season 5, Episode 19 reveals that Oboro was a classmate and friend of both Eraser Head and Present Mic back in their U.A days. He was a popular student, one who truly had the heart of a hero and was moved to protect everyone he could. Unfortunately, Oboro's caring nature proved to be his undoing. He died in the line of duty some pushing himself too hard, and until now, that was seemingly the end of his story.
When Eraser Head and Present Mic prove Kurogiri is actually Oboro in the episode, they inadvertently confirm a gruesome truth: The Nomu are made using human corpses. All For One coveted Oboro's Quirk and stole his body in order to mix other Quirk factors with it. The result was Kurogiri's Warp Gate. The reanimation process completely wipes the person's memory, leaving only their strongest instincts behind. In Oboro's case, that was his protective nature, which Kurogiri now directs at Shigaraki.
Everyone watching Erased Head and Present Mic's interrogation of their old friend is equally -- and understandably -- horrified upon learning the morbid truth of the Nomu. But the similarly captured All For One seems to take pride in his obscene science. He likens the creation of Nomu to the wine-making process, comparing the extraction of Quirks from corpses to the grapes that get crushed to produce wine.
Seeing as several different people are required to create a Nomu, making just one is a horrible enough crime. But of course, ethics don't matter to MHA's big bad. He's created quite a number of them already and doesn't plan on stopping anytime soon.