Double

Rendered Nibai in Japanese and dubbed Twice through the anime's third and fourth seasons, Double was a contact-range Emitter Quirk that surfaced in chapter 77 and episode 43. Jin Bubaigawara owned it; the Nomu Mocha carried a manufactured version.

The premise was simple to state and enormous in practice: touch something, produce a copy of it, two at a time. Aimed at other people that meant reinforcements out of nothing; aimed at Jin himself it meant an army, and an identity crisis he never escaped.

Jin worked by pushing a dark, thick fluid from his hands, which gathered into the shape of whatever he was reproducing. A replica arrived with the original's personality and abilities intact, near enough to pass a first glance, though its recollection reached back only to its last encounter with Jin. Creating one demanded exact data, precise measurements and physical detail held in mind; anything less produced a malformed thing that fell apart quickly, and any bodily change meant measuring the person again. Objects also required knowledge of their composition and structure, without which the copy decomposed. Skeptic believed the ability could reach even the dead.

The ceiling of two was a technicality. Because a double of Jin also possessed Double, each copy could make copies of its own, and so could those, so the count climbed without real limit as long as the chain held. He built himself far faster than anyone else, and a half-wrecked double kept multiplying provided its arms survived. Giran reckoned the power could hand Jin the entire country.

Fragility was the counterweight. Copies came out weaker and less durable than the genuine article, and damage on the order of a broken bone typically finished one, leaving it slumped into viscous mud. Each inherited its template's condition at the last measurement, so a duplicate of a mortally wounded Himiko Toga arrived mortally wounded. Jin felt nothing when a double died but registered the loss at any distance, and Erasure failed to dissolve them. The sharpest limitation was autonomy: doubles thought and acted freely, owed no obedience once they knew what they were, and could turn on their maker.

Jin Bubaigawara's history with the Quirk is inseparable from his unraveling. Friendless, he first made copies for company, then drifted into theft and robbery alongside them. Treating them as servants was the error: they shared his lazy, overbearing temperament, resented the labor, and revolted, tying him to a chair. Each believed itself the genuine Jin, and the brawl that followed ran nine days. When it stopped he no longer knew whether he had survived or a clone had replaced him.

That left him afraid to copy himself, and he claimed too long without his mask would split him apart, a line that may be literal or may stand in for his dissociative identity disorder. The Meta Liberation Army Arc broke the spell: wounds taken protecting Himiko Toga proved he was flesh rather than facsimile, and he began duplicating himself again, finding gentler uses too, such as a Himiko clone supplying a blood transfusion. To the League of Villains he was a numbers advantage and a shield, sending expendable stand-ins where real members would have been arrested; Shigaraki treated him as essential, and the Meta Liberation Army moved to recruit him.

Mocha, a Nomu, carried an artificial version whose limits were never established. He extruded the same fluid from his ears to duplicate Kyudai Garaki, who sent the copy to cover his rounds at Jaku General Hospital while he operated on Tomura. It melted once another Nomu drilled through its arm. Himiko Toga inherited the power at one remove after Dabi handed her a vial of Twice's blood, letting her Transform Quirk run Double: Sad Man's Parade, the self-multiplying swarm Jin made famous; Sad Man's Death Parade, a rage-driven escalation in Twice's shape; and Sad Man's Legion, clones wearing the faces of Heroes and Villains whose blood she gathered at the Gunga Mountain Villa ruins.