The notion of holding multiple quirks was initially an alien concept in My Hero Academia’s hero world, especially since perfecting even one quirk was seen as a demanding feat. However, the introduction of Nomu, artificial humans infused with multiple quirks, brought about the concept of one individual bearing more than one quirk. It wasn’t long before humans with the power were introduced, making multiple quirks a standard format.

The power of bearing multiple quirks is often discussed, yet the crippling drawbacks that accompany this power tend to get sidelined. With several humans now possessing multiple quirks in My Hero Academia’s narrative, the taxing nature of holding more than one quirk has become quite apparent. Be it physical strain or mental regression, the side effects of multiple quirks showcase exactly why possessing more quirks isn’t equivalent to more power.

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The Source of Multi-Quirks

Possessing more than one quirk isn’t a natural phenomenon, as humans can naturally only manifest one quirk. The concept of multiple quirks originates from two different sources, One for All and All For One. All For One can steal other's quirks and grant his subordinates those powers, essentially turning them into multi-quirk individuals. Nomu were also created based on this concept, with All For One providing the quirks for these artificial humans.

While One For All works on different principles, it can be passed on to successors, and if those next-in-line already possess a quirk, they will essentially possess more than one quirk. Moreover, the quirks of all predecessors are also accumulated inside One For All, naturally turning new wielders of this power into multi-quirk individuals.

The Negative Effects of Multiple Quirks

Despite being a source of immense power, possessing multi-quirks is accompanied by grave effects that can’t be ignored, even outweighing the benefit of bearing more than one quirk. Since Nomu are artificial beings created out of human corpses, their bodies are engineered to withstand the stress of multiple quirks. Human bodies, on the other hand, don’t have the luxury of bioengineering to withstand the strain of more than one quirk.

One For All wielders and All For One’s subordinates who have received multiple quirks prove the notion that human bodies begin to cripple with the addition of extra quirks. Taking on a new quirk can be strenuous in its own right, as seen in Deku’s case when he became the successor of One For All, yet the effects of multi-quirks go beyond mere physical debilitation.

Extreme Physical Strain

Quirks that result in physical mutations or have accumulated power can cause a great deal of stress on the host. One For All is a prime example, as it can even cause the wielder’s body to explode if it’s not strong enough. The strain is also dependent on the quirk factors that accompany the power.

Spinner’s new quirk, Scalemail, resulted in immense physical stress, causing his body to bleed the more he used the quirk. Gigantomachia is the only known multi-quirk user to resist the physical strain of bearing more than one quirk, though this is largely due to his endurance-boosting quirks, coupled with bioengineering by Dr. Kyudai Garaki.

Mental Debilitation

The most prominent adverse effect of multiple quirks is mental debilitation, apparent in almost every single multi-quirk user. Gigantomachia is a prime example of this drawback, as boasting seven quirks rendered him a mindless monster with no free will of his own. While Gigantomachia had some resistance to these drawbacks, others underwent similar effects even with one or two extra quirks.

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Spinner’s mental state had greatly regressed after receiving Scalemail and Body Bulk from All For One, even losing his mind, similar to Gigantomachia. It isn’t clear whether the mental regression of Nomu is also a result of possessing multiple quirks, though it’s likely since these artificial humans are technically alive, yet they lack the ability to think, similar to Gigantomachia or Spinner’s mental state.

Reduced Lifespan

The strain of possessing multiple quirks goes beyond mere physical strain, as it can eventually lead to a significantly reduced lifespan. One For All is known for leading its users to an early death, owing to the immense strain of merely possessing this power. However, this is only the case for those users that already possessed a quirk of their own, meaning all of Deku’s predecessors became a victim of One For All’s strain.

Since Deku was initially quirkless, his lifespan largely remains unchanged, though the One For All’s stress would likely have some sort of ramification. It is unknown how fast One For All causes its users’ bodies to decay, though it is believed the user’s lifespan will be reduced by half considering the fourth wielder of the quirk died around the age of forty. This drawback isn’t limited to One For All, as possessing more than one quirk of any sort will lead to a shortened lifespan due to the extreme physical and mental strain.

My Hero Academia is available to stream on Prime Video.

My Hero Academia

In My Hero Academia, Young Deku is born into a world where people commonly have something called quirks, also known as superpowers. He himself is born without any, but circumstances lead to that changing. He enrolls at the UA High School, a prestigious academy where the worlds future heroes learn how to use their powers.