The My Hero Academia anime's power system is pretty easy to understand. In their world, about 80% of the population have meta-abilities called Quirks. These Quirks manifest early in life and mature along with the individual. If they're left unused, they maintain their level of strength but constant practice and training will result in a steady growth in the Quirk's strength. One little-known secret about Quirks is that in special cases, they are capable of a sharp increase in strength. This is termed a Quirk Awakening.
Quirk Awakenings are a very rare phenomenon impossible to achieve by training alone. For a Quirk to awaken, its user has to be placed under extreme stress, whether physical or mental. A person or their loved one being placed in real or apparent danger is sufficient grounds for a Quirk awakening, as well as them being placed in a life or death situation or unlocking traumatic memories. As Japan's crime rate was the lowest it had ever been due to All Might, it's no surprise Quirk Awakenings were uncommon, even among the Pro Heroes.
A Quirk that is awakened usually grants the user access to new powers already related to their Quirk or a boost in the abilities of their already existing Quirk. This boost comes with no risk to the user, instead often giving them a burst of energy that allows them to explore their newly awakened powers and escape the situation that forced their evolution. Currently, in the My Hero Academia anime, three characters have awakened their Quirks: Tomura Shigaraki, Himiko Toga and the Meta Liberation Army member Geten.
Tomura Shigaraki
Tomura Shigaraki's Decay Quirk was always high on the list of MHA's deadliest Quirks. Any object he touches with all 5 fingers instantly crumbles to dust. Its effect is universal; Whether an object is organic or inorganic, they all decay the same, and even the most polished of metals aren't immune to its effect. When he awakened it, his Decay was instantly catapulted to a new level of danger.
Shigaraki awakened his Quirk during his fight with Re-Destro. As the two fought it out, memories of Shigaraki's childhood began to return in bits and pieces. The return of his traumatic memories along with the unrelenting force with which Redestro pursued him provided the perfect catalyst for his Quirk to awaken.
The awakened Decay removed all previous limitations on his Quirk. Even with two of his fingers destroyed, Shigaraki was able to activate his Quirk for a moment. He attempted to begin crumbling his foe but Redestro flung him away before he could really get started. The five-finger activation requirement was not the only limitation he overcame. Before activation, Shigaraki's Decay was limited only to objects he was directly touching but his awakening allowed it to spread unchecked. His Decay became able to spread to any object his initial target was in contact with as well as whatever was touching the secondary object too.
His Quirk became able to spark an unstoppable chain reaction of destruction that could spread as far as Shigaraki wanted it to. The shockwave he created right after awakening spanned several city blocks. But since he was weakened by fighting Gigantomachia, the Meta Liberation Army, and Redestro prior, it's possible Decay's true area of impact could be even larger.
Himiko Toga
Himiko Toga's Transform Quirk made her the perfect covert-ops agent. After ingesting someone's blood, she could transform into them for a period of time proportional to the amount of blood she used. She could become an indistinguishable copy of anyone and if she'd ingested the blood of multiple people, could switch between them at any time she pleased. Probably the only way of recognizing a transformed Toga was confirming a person’s Quirk. Toga couldn't use anyone else's Quirks. Her transformation was only skin deep but her Quirk awakening removed that limitation entirely. She awakened her Quirk while fighting for her life against Miss Curious, another general of the Meta Liberation Army.
Toga had transformed into Uraraka Ochaco to evade capture but Miss Curious spotted her anyway. As she drew nearer to Toga for the finishing strike, Toga's fingers brushed her, and to both their surprises Miss Curious started floating. Realizing she now had access to Ochaco's Quirk, Toga quickly floated all her assailants and then released the Quirk when they were high enough, killing them all when they dropped instantly.
Toga became able to use other people's Quirks implied that her transformations were no longer skin deep. Quirks are tied to a person's genetics and her transformations allowing her access to them means she has now become able to change her very genetic makeup. This makes Toga all the more dangerous because there is now no way to tell who she might have transformed into. Biometric identification wouldn't work against her because for all intents and purposes, she now becomes whoever she's transformed into.
Geten
Geten's Quirk Awakening might not have been as a result of a situation as dramatic as the other two but the ability it gave him was no less powerful. Geten's initial Quirk was the ability to telekinetically control ice but after awakening, he became able to control all of its aspects, including its shape and temperature.
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Geten awoke his Quirk off-screen, in an accident when his leader and mentor, Redestro, burned himself. It wasn't a particularly dangerous incident but it being enough to awaken Geten's Quirk speaks to just how devoted he is to both Redestro and the Meta Liberation Cause.
Being able to change the temperature of ice doesn't seem like much of a step up from controlling it but it was the perfect boost for Geten. Ice isn't exactly very common but the ability to control its temperature gives Geten an almost unlimited supply. Even with a single cube, the ability to lower its temperature means dropping it into any water body will result in more ice. Being an urban villain, the water mains in any city guarantees Geten constant ammunition for his Quirk.
