Welcome to the Quirkipedia! Due to quirks tending to be a little complicated, I thought it good to compile a list of my own creation. Each quirk will have it's own short chapter and they will be uploaded when and in order of appearance in the main story, 'Vigilantes or Thieves'. This is not required to understand the quirks and the quirks will be explained and explored diagtetically in the story anyway, so don't feel any need to read this aside from a want for a little more information.
Quirk: Metacognition
Wielder: Ren Amamiya AKA Joker
Ren Amamiya. Known to a select few as the leader of the infamous Phantom Thieves, he has one of the most powerful and versatile quirks in their repertoire. Metacognition allows the user to sense, manipulate, reificate, and exploit the subconscious mind of individuals and the general populace. Things that people believe are possible or true tend to be true in Ren's presence (to a certain extent). One of his simplest and most common exploits of the subconscious mind lies in the average person's fear of weaponry - as long as the general populous holds a fearful disposition towards deadly weaponry Ren's lifelike model pistols and knives will feel as painful as the real thing.
Because of his ability's basis in Metacognition Joker's weapons and skills come with a few quirks of their own. Firstly, his pistol's gunshot is only visible and audible to those who recognize that the gun is being fired - if someone in the room next door has no idea of what is happening they will hear nothing. They have no cognition of the weapon - therefore, to them, it doesn't exist. Similarly, if someone had no cognition or expectation that Joker is hiding in the shadows it's far more difficult for them to spot him. Secondly, as they are only models, the gun produces no bullets and the knives make no cuts. People cannot be physically hurt by his weapons - but if they believe they have been shot or cut, they and any witnesses nearby may temporarily see blood and wounds. Similarly, anyone who expects to be hurt will feel as though they were injured - and for many otherwise deadly shots are enough for their system to panic, overload, and fall unconscious. These traits lend well to silent and nonlethal takedowns. He's also able to effectively utilize his subconscious awareness to help better detect someone's lies, sense a nearby person's presence without seeing them, and even subtly improve some people's disposition towards him.
These same oddities can lead to many difficulties, however. If the gun only works based on awareness how could it be helpful when targeting someone who isn't aware of the attack, or when going against someone impervious to bullets? Fortunately, Ren has an answer to this as well. As stated before he is not only able to exploit cognition, he can (in minor ways) manipulate it. Imposing the general populous' fear on a normally bulletproof enemy would work, as would psychically alerting an unaware enemy of the danger the same instant they're attacked. These are only temporary fixes, however. Imposing or manipulating cognition is extremely difficult, and taxing and often lasts only brief periods. Joker has often found other ways around these limitations and has found success in manipulating someone's cognition in more traditional, non-quirk-related ways.
Finally, Metacognition has one glaring weakness. As he can affect cognition, cognition affects him. One of the most annoying byproducts of this is that while he may be sneakier in a place that is generally viewed as unsafe or where he is entirely unexpected, he is psychically repulsed by places where people believe he cannot enter or where he holds no power. In places where people cannot begin to imagine someone sneaking around, or if his enemy believes him to be inept or in possession of pitiable weapons, he will find himself at severe disadvantages.
Of course, like all quirks, Metacognition is more than it seems. Every day it seems to grow, and every day he seems to find a new application of this strange and versatile ability. Even with all this, Joker's true strength might lie more in his actual skill - quick, strong, quiet, dexterous, and smart, he keeps up with and surpasses his sometimes flashier teammates.