WARNING: The following contains spoilers for My Hero Academia Season 5, Episode 12, "The New Power and All For One," now streaming on Crunchyroll, Funimation and Hulu.
The heroes have the edge over the villains for the moment in My Hero Academia. The criminal mastermind Overhaul has been defeated and young Eri was rescued from his clutches. Eri is no ordinary child, either. She has a Quirk named Rewind, and it is a burden, to say the least. Shota Aizawa/Eraser Head is desperate to find a way to fix this.
The Joint Training exercises have just wrapped up, and during the fifth match, Class 1-B's loudest member, Neito Monoma, faced Izuku Midoriya in direct combat with his unique Copy Quirk. This inspired Aizawa to use Neito's talents to help Eri, but there's a surprising new obstacle in their way: the "blank Quirk."
Although Neito was born with his own Quirk, it does nothing at all if he is by himself. The power, known as Copy, allows him to imitate and use the Quirks of anyone whom he touches bare-handed. Neito has previously borrowed some Quirks and put them to good use, such as Eijiro Kirishima's Hardening ability and Katsuki Bakugo's Explosion Quirk halfway through My Hero Academia's U.A. sports festival arc.
In theory, he can copy any Quirk, but he does have preferences. Ideally, he will copy one with simple but powerful applications because a tricky Quirk would take a while to train with. Neito doesn't have the time to borrow a complex Quirk and try to master it in the middle of battle. Thus, simple and convenient Quirks are best.
However, as Neito is now finding out, a few Quirks are actually "blanks," in his own words, and are impractical to copy no matter how desperate he might get. Izuku had never heard this term before, but Neito generously explains the theory. A "blank Quirk" is a Quirk factor that requires the user to gradually build up a certain resource to fuel its power. An analogy would be if Neito copied a car but only the vehicle itself, with no gasoline inside. He cannot refill the car's gas tank, so to speak, and thus the power does no good in his hands. Neito must borrow more than just a shell of a Quirk.
A majority of Quirks seen so far in My Hero Academia would be usable for Neito, but the legendary One For All and Eri's Rewind ability are both categorized as blanks -- along with, in theory, Fat Gum's fat-absorption Quirk. Such Quirk factors all require energy and fuel to work, and Neito cannot borrow that when he uses Copy. He copied One For All during his fight with Izuku, but it was empty since One For All is, above all else, a power-storage Quirk.
In fact, one of the previous users spent his entire life just stockpiling One For All's energy for the benefit of a future user. Izuku can access that energy, but Neito cannot use and discharge energy that he doesn't have. Thus it's a blank, and Neito doesn't have time to start charging up. Besides, One For All can break the body of an unworthy wielder, and Neito has not undergone the proper training.
The same is true for Fat Gum's Quirk, which allows him to absorb kinetic energy with his body fat and convert it into sheer power for his muscles at will. Neito can copy that Quirk factor, but he'll have zero kinetic energy saved up, and has far too little body fat to stockpile energy the way Fat Gum does.
Eri's Rewind Quirk also requires a huge stockpile of energy, and even if Neito copies it, he wouldn't borrow the saved-up energy, thus her Quirk is also a blank. Alas, Eraser Head cannot use Neito's talents to imitate Eri's Quirk and restore Mirio Togata's own. Perhaps My Hero Academia has another solution out there, still waiting to be found.
