Wielder: Morgana AKA Mona
Subcognition is similar in many ways to its brother Metacognition. While similarly tied to the subconscious mind and people's perception of the world, Subcognition, as its name might imply, delves deeper and stranger. Subcognition gives its wielder and those its wielder chooses the ability to sense, explore, comprehend, and exploit another person's subconscious mind. It also gives its wielder average human cognitive abilities, although it's difficult to say if that is the quirk's direct doing or a byproduct of Morgana's exploration into the human mind. Its most prolific application is in its use by the Phantom Thieves to enter and affect a sufficiently vulnerable target's cognition and to shape that cognition in a way that its visitors can comprehend.
Subcognition is even less defined and, in some ways, more powerful than its sibling. Directly interfacing with a person's subconsciousness can lead to permanent damage, personality changes, and even death. Morgana's experience in these "palaces" allows him to exploit these risks into purposeful changes which allows the Phantom Thieves to infamously "Steal the Heart" of various criminals and villains.
It has been found that the more powerful a quirk the more specific and conditional it becomes - or perhaps it is the other way around. Subcognition is no different. It can only be used to enter into a sufficiently powerful cognition, one that is vibrant and solidified. Typically the only cognitions that are powerful enough to enter are those of people with distorted views of the world - extreme beliefs and comprehensions that cannot be broken even from direct and clear evidence against them. Even when a palace exists, it cannot be so easily entered. A person typically thinks of themselves and their mind as the most secure place they can be - and so, of course, it is. Only when the Phantom Thieves challenge this security and safety to a sufficient degree are their cognitions able to be properly entered and explored (although, Morgana can bring others to the edges and weaker parts of a palace even before this).
Evidence of crimes the perpetrator believed to be unrecoverable, a mysterious message left in their most private sanctum, or even a change of heart brought upon a friend or rival are all examples of shattering that security and opening a person's cognition to infiltration. In this, most of a Phantom Thief's work is done in the real world.
Due to the aspects of this quirk, it means that in physical reality Morgana is little more than a cat. In combination with his own and Ren's quirk, he can speak to those who know of his intelligence but little else.
When entering into a person's cognition, dubbed the "metaverse" by the cat, the physical forms of the Phantom Thieves disappear from reality. After traversing through a person's palace they are only able to return to the real world by the same area they entered.