Steel allows Tetsutetsu to turn his skin into a steel-like substance.
Steel makes Tetsutetsu's body virtually impenetrable; it is tough enough to protect Tetsutetsu from bullets, blades and tons of falling metal, as well as other lethal hazards. Naturally, it increases his physical strength to superhuman levels, too, allowing him to break through most obstacles. Tetsutetsu has said that he will gain longer and stronger use of his Quirk depending on the level of iron that is in his diet. It is unknown if this refers to foods that have iron in them or if he has to eat the actual metal itself.
Steel can protect Tetsutetsu from extreme temperatures. Ice and cold have no effect on his skin when Steel is activated, making Tetsutetsu immune against freezing. Steel may also grant resistance towards high heat, however, Tetsutetsu has noted that he went through Quirk training in order to achieve that feat, implying that Steel is not naturally effective against fire. His steel body gains a red coloration if exposed to flames.
Using his Quirk for too long will cause Tetsutetsu to suffer from iron fatigue, gradually weakening Steel's hardness. Steel doesn't protect Tetsutetsu from all forms of damage, as poisonous gas can still be effective. Tetsutetsu has noted that Steel has a set limit of hardness that cannot be improved upon, unlike Hardening.
According to Juzo Honenuki, Tetsutetsu's body might melt if subjected to a certain degree of heat.
Very much like Eijiro Kirishima, Tetsutetsu is extremely straightforward and reckless, taking fights up close and letting his Quirk protect his body from harm as he dishes out counterattacks. Also, similar to Eijiro, Tetsutetsu is able to turn himself into a human shield, blocking attacks meant for his allies.
Tetsutetsu had to undergo training in order to become immune to high temperatures.
Named Super Moves
- Steel Fist (俺拳 Ore Kobushi?): Tetsutetsu rushes towards an opponent and delivers a simple jab while in his Steel form. It's strong enough to shatter ice. This move is similar in nature to Red Gauntlet. First used against Shoto Todoroki during the Joint Training Arc.
- Horn Dash Hammer (角ダッシュハンマー Tsuno Dasshu Hanmā?): Combined attack between Tetsutetsu and Pony Tsunotori. First Tetsutetsu turns his body into steel and then, thanks to her Horn Cannon Quirk, Pony holds him through her controllable horns and launches him towards an enemy. First used against Shoto during the Joint Training Battle.
- This Quirk is very similar to Hardening, and the two appear to be roughly equal in terms of power and effectiveness.
- The effects of Hardening and Steel are very similar, but the latter Quirk turns the user's entire body into hardened steel instead of just hardening their skin. The two appear to be roughly equal in terms of power and effectiveness, although Tetsutetsu notes that his body can only become so hard due to transforming into metal, whereas Eijiro can become increasingly tougher.