My Hero Academia's Eijiro Kirishima, AKA Red Riot, is a straightforward guy with a straightforward Quirk -- or so it appears. Despite the seeming simplicity of his Hardening Quirk, it hides a few surprises and secrets -- and likely even Kirishima himself doesn't know all of them. Here are five strange and noteworthy facts about Kirishima's Quirk.
Kirishima's Hardening Makes Him Literally Sharp
More than just defense, Hardening allows him to go on the offensive as well. From the very first time he manifested his Quirk, the sharpness of his rock-solid form has been noted as capable of cutting regular human skin. While he originally cut his own eyelid (giving him a scar) this trait has continued to carry over into his U.A. career. This means his punches deal two types of damage, blunt-force and sharp-force, as proven when he nicks Bakugo's cheek during their sports-festival fight. This effect is only heightened when he's in his unbreakable form, where his fingers become claws. These sharp edges can deal massive damage to enemies, draw maps into hard surfaces and puncture walls to aid in rescue scenarios.
The sharpness of his Unbreakable form can also be used as a scare tactic, similar to how certain animals will puff out their fur to look more imposing. While this probably won't work against overconfident villains, against lower-level opponents, it may help to avoid a fight. Even if they do manage to land a hit, it'd be like punching a craggy rock face, likely causing them to break their own bones and tear their skin against Kirishima's rugged exterior. Either way, it's gonna hurt them a lot more than him.
Kirishima Hardening Allows Him to Vary the Degree of Hardness Across His Body
Unfortunately, Kirishima can't keep up maximum hardness for indefinite periods of time -- it takes energy and stamina to keep his hardened form together. But there are some clever ways Kirishima can make use of Hardening to keep his stamina up. Firstly, since he can choose what areas of his body are affected by Hardening, he can easily harden just the area the attacker is about to hit, maximizing his own damage resistance and the damage impacted to the attacker by making contact. Secondly, he could easily "shift" the area with maximum hardness around his body, leaving the rest of the body at standard hardness to prevent massive damage from surprise attacks while still making sure he's actively blocking the more dangerous attacks.
He can make great use of this to enhance his mobility and rescue abilities. While he can still move with his Quirk activated, by leaving his legs unhardened, he'd greatly lessen the stamina required to keep the rest of his body hard and take advantage of his opponents' wrong assumptions that his whole body is affected. Similarly, while he starts wearing sleeves during the Provisional licensing exam to protect the people he's rescuing from his sharp cutting skin, he can depower his arms while still keeping his head and torso hard for maximum protection.
Hardening Lets Kirishima Withstand Every Type of Physical Trauma
One of the first times that the audience is introduced to Kirishima in any meaningful way is when he volunteers to join Bakugo during the Calvary race at the U.A. sports festival. The main reason he's able to volunteer to join the volatile Bakugo so readily is his Quirk, Hardening protects him from every kind of physical trauma, including explosions. Kirishima has been shown to be able to take a direct hit from Bakugo even this early in his heroism career, and while he did eventually lose, one thing remains obvious. Any normal person would have been at least singed. Just like the rocks lining a fire, Kirishima's hardened exterior makes him impossible to burn and allows him to withstand extreme temperatures.
But Kirishima's immunity isn't a one-trick pony. Cutting blows glance off his skin, and blunt-force-trauma only serves hurt the attacker. Not even bullets can pierce his skin, which is notable as bullets inflict an entirely different type of trauma than both cuts and blunt-force. Arrows and other projectiles just bounce right off, making him the ultimate shield. However, that doesn't mean the attacks don't hurt, and this early in his career, he does still have a breaking point, whereby he can lose his hardness, once again making him vulnerable to all types of trauma.
Kirishima's Hardening is Only Skin-Deep
Kirishima's Hardening only affects his skin, eyes, hair and bones. The first attack Rappa lands on him during the Hassaikai arc sends him flying into a wall, breaking his Hardening so badly that his skin literally cracks off, revealing the red muscles underneath. Later in the fight, he's shown to be bleeding from various crevices in his skin, and the blood is very much still a liquid. While we don't know how many of his internal organs are affected, since his teeth harden with the rest of his exterior, it's only safe to assume the rest of his bones do too. Either way, once his defense is cracked, a trip to the hospital is most definitely in order until his skin regrows.
There's No Limit to How Hard Kirishima Can Get
Much like muscles, once broken, Kirishima's Hardening comes back stronger. With every hit he takes, Kirishima increases his durability, stamina and cutting edges, as each level of Hardening makes his form more and more jagged. Just like any good hero, Kirishima proves it's not how many times you get knocked down, it's how many times you stand back up, and his Quirk perfectly suits this straightforward boy by echoing that inner strength physically.