The Quirks of My Hero Academia Part 3

The Quirks of My Hero Academia Part 3

Both the My Hero Academia Manga and Anime are going strong with no end in sight, and Kōhei Horikoshi continues to add characters with interesting quirks.

Good Ear- Sirius


Sirius is an anime original character that first appears in Episode 27. She is a pro hero that works under the pro hero Selkie. Her quirk is called good ear and as the name implies her hearing is much better than a normal human. I’ve already discussed the basics of how human hearing works on my post on A Silent Voice, but here is a short recap.

Sound is collected by the outer ear and funneled into the middle ear. The sound hits the tympanic membrane at the end of the middle ear causing it to vibrate. The vibrations are transmitted to the cochlea via the ossicles.


The vibration of the ossicles causes fluid inside the cochlea to vibrate and those vibrations are detected by tiny hairs inside of the cochlea. The hairs in different regions of the cochlea vibrate at different sound frequencies.


A normal human has a hearing range of around 20hz-20,000hz. Hz stands for Hertz, which is a measure of frequency, or how many times a sound wave repeats in a second. The higher the frequency the higher the pitch of the sound, like the high frequency mosquito sound that some stores use to deter loitering.

Elephants can hear frequencies as low as 7hz, and their ears have a similar structure to our own eccept in one area. The elephant cochlea is more tightly coiled, which helps to amplify low frequency sounds, making them easier to hear. On the other end of the spectrum, bats and dolphins can hear up to 100,000 hertz. Again, the basic structure of their ears is not all that different from our own, but their cochleae are longer. The longer cochlea allows for more hairs, and regions that can pick up higher frequency sounds. In the case of Sirius, who can hear both low and high frequency sounds, I think that her cochlea is longer and more tightly coiled than a normal person’s ear. This would allow her to hear both low and high frequency sounds and still fit in a normal human skull.


Additionally, her brain would be slightly different from a normal human brain. The temporal lobe is the area of the brain that processes auditory information. With the additional auditory information her ear is picking up, the temporal lobe would be larger in order to handle it. The alternative is that some of the auditory information is sent to another area of the brain for processing, which occurs in people who have lost their vision. The enhanced hearing they have is due to part of the occipital lobe, which normally is used for vision, being used to process auditory information. Either way Sirius’s brain is larger than average or her vision is below average because part of her occipital lobe is being used by her ears.

Yu Takeyama- Gigantification


I know that I have already covered Mt. Lady once before, but I wanted to expand on her quirk a bit. Previously I covered how she would probably suffocate from her lungs not working as efficiently at a large size. This time I wanted to highlight some other changes the gigantification quirk would cause. When it comes to increasing body size, there are two ways to do it: increase the size of existing cells, or increase the number of cells.

If the gigantification quirk increases the size of the body cells, how much of a size increase is occurring? At her normal height Mt. Lady is 5’4”, which increases to 67’7” in her giant form. This means her body is 12.5 times bigger in her giant form and this applies to all of the cells in her body.

Giant sized cells

Skin 0.375mm

Blood 0.103mm

Hair 1.25mm

This means that we would be able to see the cells that make up Mt. Lady’s body without any assistance from a microscope. Additionally, this is much bigger than normal, causing the cells to suffocate from a lack of oxygen and nutrients, as diffusion and osmosis would take too long to move material in and out of the cells.

The second option is that gigantification increases the number of cells in Mt. Lady’s body. In this case her body size increases by 12.5, and her weight increases by a factor of 1,953, meaning that she weighs 126,953kg or 279,297lbs. Surprisingly, she will be able to stand without breaking her own legs, as human bone can withstand 170Mpa (a measurement of pressure), and her weight will put 20.2 Mpa on her femurs. But bone is weaker to shear forces so a kick in giant form is probably out of the question as the landing stands a good chance of breaking her legs.

Mt. Lady

Eijiro Kirishima and Tetsutetsu Tetsutetsu


I can’t say much about either of their quirks but I wanted to highlight that despite the great protection their hardening and steel quirks provide them, there is one thing they can’t protect against: brain injuries, specifically concussions. Even if their quirks prevent an object from breaking the skin, that doesn’t prevent the shockwave from the impact entering their bodies. This is shown when they knock each other out during the tournament, and when Tetsutetsu was shot by Mustard. Even though the bullet bounced off his steel skin, the shockwave of the impact is shown to cause him pain. So while they have an excellent defense, they still need to watch out for traumatic brain injuries.

Fourth Kind- Quad arms


Quad arms is a relatively mundane quirk that gives 4th Kind 4 arms instead of the human standard of 2. There is a lot that could be said about having 4 arms and I have discussed this previously in my Evolution in A Centaur’s Life post. Given how biology is played with in My Hero Academia I can’t think of any particular downside to having this quirk.


I know it was only a few this time, but I hope you enjoyed another round of the science behind the quirks of My Hero Academia.

FYI- If you want to know how Midnight’s quirk works, it’s basically a variation of a Succubus which I discussed here.

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