Truesight

Alters the user's sensory faculties, causing them to perceive everything 'objectively'.

What this means is that they can see things as they actually are. This includes the original form of shapeshifters/Transformation Quirks, illusions, invisibility, and even learn someone's true name (related to New Order's whole naming convention).

Since this is a Mutation Quirk it cannot be turned off, so the user is always seeing objective reality. The sensory input is always being uploaded to their brain so long as they're looking at something the Quirk considers a target, which can be quite overwhelming after a while.

Closing their eyes merely causes the Quirk to look at its own user and give them insight into themselves (which is actually how they learned of the full extent of the Quirk's abilities).

It would be impossible to describe what the user is truly seeing, like trying to explain a new colour. For instance, if they observe someone morph into a dragon, they would simultaneously see the dragon form, the original human form, the Vestige, and the individuals true name all at once, sort of superimposed over one another yet still capable of differentiating them.

The only real way to suppress this effect is to put the simplest stimuli blocking the users line of sight. A blindfold works best, as the Quirk can still somewhat penetrate the material while still occupying their focus. They would have to switch out these blindfolds periodically as Truesight would recognise the same blindfold after a while and stop registering it entirely.

The worst opponent for the user is someone capable of producing many tricks in the shortest time possible. For instance, if they are fighting a shapeshifter and their enemy starts rapidly cycling through different forms Truesight would try to map out and remember all of them, which would end up overwhelming them faster.

Inspiration: Six Eyes from JJK + Truesight from D&D + the general idea of 'true forms' and seeing through someone.

User: originally some random unnamed vigilante or underground hero that was targeted by All For One. It ended up being one of his favourite Quirks since he can turn off mutations unlike its OG user, and it helped him keep track of his own Quirks. Ultimately it would be destroyed after All Might gave him the head splat (not that it would've been much of a difference with no eyes).