WARNING: The following contains major spoilers for recent manga chapters of My Hero Academia.
At the heart of Kōhei Horikoshi's My Hero Academia lies Izuku "Deku" Midoriya. Originally Quirkless (meaning, he was born without superpowers), Izuku eventually gained social abilities by swallowing the hair of #1 global hero All Might, leaving him blessed with enhanced strength and reflexes thanks to the powerful Quirk known as "One for All."
Through several trials that have typically led to him breaking his limbs or ruining his body, Izuku has gotten a better handle on his Quirk, resolving to grow stronger and fully master it like his predecessor. In an unexpected move during the latest chapter, Izuku finds himself changed yet again by One for All.
Over the course of the series, especially in more recent chapters, Izuku has displayed a mental connection to the previous users of One for All; because it's iterative and built upon the strengths and Quirks of each user, in his mind he's been seeing several figures who at one point in time possessed his Quirk.
It all came to a head in the preceding two chapters, where Izuku goes chaotic during a joint class training exercise and starts unleashing black lightning that he can't control. It's only thanks to his classmates' quick thinking that his mind goes blank, and that's when things get weird.
Again, big spoiler here.
In Chapter 213, Izuku discovers that the mental connection he's had with the previous users is actually his mind going inside One for All. Surprisingly, another of the previous One for All users starts to talk to Izuku, explaining what's going on.
Up until this point, the only other person to be able to speak to him within One for All was its original wielder, but the situation has forced this new user into the forefront. The user tells Izuku that it's his Quirk being accessed, resulting in lightning being unleashed -- the "Black Whip" Quirk -- in response to Izuku's thought of capturing a classmate.
And that's where we get our big reveal: When Izuku swallowed that hair at the start of the manga, Izuku didn't get one Quirk, he got nearly 10.
That's a pretty big bomb to drop on Izuku and the world as he -- and we -- know it. In a fit of irony, Izuku has gone from Quirkless to being one of the most dangerous and powerful people on the planet as a combo superhero. Currently, he's only activated three of the 10 powers: One for All, being able to stockpile and unleash power into his body without pain (what was originally thought to be One for All when he got the power from All Might), and the new Black Whip. To make things even stranger, it's a lot stronger in Izuku than it was with its original user, which likely explains why he nearly destroyed his arms when using powered-up punches.
Most humans in the world of My Hero Academia have some type of Quirk, and it would be strange if every user of One for All was Quirkless like Izuku and All Might were. One for All's official history is that it stockpiles other Quirks in addition to transferring power, essentially an inversion of series' antagonist All for One and the Quirk he takes his name from. All for One hoards Quirks for himself, while One for All's various users gift them to their successors.
If there's any positive from this, it's that Izuku has a kindred spirit in fellow multi-Quirk user and classmate Shouto Todoroki. Having multiple Quirks isn't something that can be easily hidden, and much like how he consulted his friend Iida for how to use kicks without shattering his legs, Izuku can learn from Shouto to learn how to use multiple Quirks.
Izuku's way of meting out his power has been by percentages, so he's got quite a ways to go before he masters them. But the matter may be out of his hands as One for All continues to grow. This may just be what gets him to defeat All for One and become the Greatest Hero in the World... provided he doesn't break any more bones, of course. My Hero has always succeeded at taking American superhero tropes and turning them on their head; now it's time to see what it does with the newest card in the deck.