Overmodification

Overmodification is an Emitter type Quirk working by contact, held by Anna Scervino and introduced in My Hero Academia: You're Next. Where compatibility exists, it lifts the recipient's Quirk to a far higher level and, with enough contact, can add abilities outright. Activation announces itself as Anna's hair darkens to black and roses bloom out of nowhere, while the recipient's pupils briefly widen.

Modification repeats freely, with no obvious immediate strain on the person receiving it, and the results are unpredictable at first but generally severe, enough to make a simple Quirk troublesome. Deborah Gollini's Day Dream, once limited to one target at a time, ended up sweeping over crowds including Class 1-A. A rose appears on the clothing of anyone compatible. The incompatible get petals that bloom and then fall apart, followed by pain fierce enough to knock them out. What determines compatibility is unknown, but it is extraordinarily uncommon: of the twenty thousand the Gollini Family once counted, eight came through the process with boosted Quirks.

The Quirk never rests. Only a cancelling ability such as Neutralization stops it, and it throws off plant growth wherever it goes, even on barren ground. Anna controls none of this, so she either harms the incompatible or gets exploited by villains, and the weight of it left her emotionally hollow.

Holding back hurts her too. If she does not release enough of the power, it turns inward and brings on agonizing seizures that need a cancelling Quirk to break. Left unchecked, or pushed too far, marked by her hair going pitch black, the Quirk overloads and drives her toward self destruction, its range ballooning outward, driving compatible people to abominable strength and throwing out thorny vines that whip at anything nearby. Enhanced people lose their modifications under enough punishment, though the threshold scales with strength: Ginji Kurau reverted after one brief fight, while Dark Might needed Katsuki Bakugo, Shoto Todoroki, Giulio, and Izuku working in concert before most of his transmutation gave way.

Anna is the sole holder. Seizures and the harm she caused by touch kept her secluded indoors through childhood, and only her butler Giulio Gandini, using Neutralization, gave her anything resembling a normal life. As her power kept climbing, her father feared Giulio would eventually fail to contain it, and Anna extracted a promise from him: should she ever lose herself to the Quirk, he would kill her.

The Gollini Family learned what she could do, stormed the Scervino mansion, killed her father, maimed Giulio, and took her. Their thousands were culled to eight compatible members, their leader Dark Might among them. They kept her placated under Deborah's Day Dream and kidnapped civilians by the thousand as specimens, nearly all of whom simply collapsed. Dark Might boosted his Alchemy over and over until he claimed to have surpassed All Might at his peak, and after his defeat the Quirk went haywire and came close to killing her.

Eight Gollinis survived the process. Bruno's Delay Spot, once cramped, stretched to blanket a mansion yard. Paulo's Erasure Spot widened until it swallowed a mountain range. Simon's Monster Summon jumped from a modest handful of creatures to a swarm with no apparent ceiling. Ugo's Telekinesis went from light work to lifting boulders, ice, and stone slabs together. Gil's Teleportation quickened to something close to instant. Deborah's Day Dream broke free of its single target restriction. Kamile's Spatial Excursion strengthened enough to hoist the Symbol of Paradise. Valdo, better known as Dark Might, saw the ceiling on his Alchemy rise far enough to build that fortress in the first place.

Outside the family, Ginji Kurau's Mad Gluttony went from one fanged mouth to a spread of biting tendrils, and a few unnamed civilians proved compatible too. So did Giulio. His boosted Neutralization spread out from a single hand until it covered him completely, sealing Anna's Quirk Factor for good and finally letting her live normally.