Steel

Steel is the transformation-type Quirk carried by Tetsutetsu Tetsutetsu of Class 1-B. It works at close range, debuts in Chapter 25, and reaches the anime in Episode 16. Rather than coating or reinforcing him, the ability remakes his whole body as steel.

Comparisons to Eijiro Kirishima's Hardening followed almost immediately, since both trade in raw durability, and the two are generally treated as near equals in power and utility. The distinction lies in method: Eijiro toughens his skin, while Tetsutetsu becomes metal outright. That difference cuts against him in one respect, as he can only reach a certain hardness, whereas his counterpart keeps getting tougher. Outside the series, the power invites an obvious parallel to the X-Men character Colossus.

In his transformed state Tetsutetsu is close to impenetrable. Gunfire, edged weapons, tons of collapsing metal, and comparable lethal hazards all fail to get through, and the change carries an accompanying jump in physical strength that lets him plow through most barriers in his way. He has claimed that the iron content of his diet governs how long and how effectively the Quirk holds, though whether that means iron-rich foods or literal metal has never been clarified.

Temperature extremes are largely handled. Cold does nothing to his activated skin, which leaves him unbothered by natural freezing of the sort Shoto Todoroki produces. Heat resistance is a different story: Tetsutetsu has said he underwent Quirk training specifically to withstand it, implying the protection is earned rather than innate, and flames turn his metal body a reddish shade.

The weaknesses are real. Holding the transformation too long brings on iron fatigue, which steadily erodes the hardness he depends on. Poisonous gas bypasses the armor entirely, since breathing remains a necessity. Juzo Honenuki has also suggested that sufficient heat could melt him outright.

Tetsutetsu is the sole holder, and his approach mirrors Eijiro's temperament as much as his power set. He is blunt and reckless, closing distance without hesitation, letting the metal absorb whatever comes and answering with counterattacks. He is equally willing to plant himself in front of teammates and serve as a living shield.

Three named techniques belong to him. Steel Fist is a straightforward jab thrown at a charge while transformed, powerful enough to break ice, and Shoto took one first in the Joint Training Arc. Horn Dash Hammer is a joint move with Pony Tsunotori, who uses her Horn Cannon to grip his metal body and fling him at a target, also debuted against Shoto. Heatetsu takes the opposite tack, having him soak up enormous heat until he glows red, then turn that stored temperature into scalding strikes.