On Awakening, Ego Break and Hubris
Although the general in-universe knowledge of Egos stops here, this is but the first step of a path that has been long lost to time.
In truth, the power granted by an Awakening is, technically, completely malleable. The protagonist’s supposed special ability to swap Mementos at will is not special at all: if they hadn’t been conditioned to believe otherwise, every other Awakened character in the game could do that, and in fact some do reach that point because of internal turmoil or some revelation.
Maya is the main example of this, as being bound to Roxenal granted them this knowledge and allows them to summon a variety of different Egoes.
Although Ego is mostly used by the characters in the game as a weapon/ability summoning utility its real nature is completely different and far more powerful.
Ego is the power to mold and materialize the Oniric Plane into the Material one, essentially rewriting reality in a radius around the user. It’s born from exceptional desire, and thus controlling it is incredibly hard. The size of a materialized Ego can range anywhere from a couple meters to several kilometers, depending on its nature; this can obviously lead to huge turmoil and horribly destructive quasi-nukes if someone ruthless were to obtain this power, and for this precise reason its workings have been smothered by history, with active murder first and misinformation later, to the point that even most Egoes are not aware of the truth.
This, however, is a blessing in disguise.
As discussed in the Setting section, monkey brains are not great at rewriting reality on the fly, and thus extreme Egoes are almost exclusively the domain of gods and madmen.
When battling each other, however, Awakened Ones may catch a glimpse of the truth in the form of what is known as an Ego Break, a phenomenon in which the link between an Awoken and its Ego is temporarily severed and its consciousness thrown back in the Oniric Plane, which leaves its Material body vulnerable.
Despite its name, many beings who don’t possess an Ego per say can be Ego Broken: the name derives from it being far more common and useful against Awoken, as most Egoes at least somewhat protect their host from harm.
The last topic to discuss about Egoes is a horrifying consequence of their nature. Since they are able to rewrite the world itself, and their user is part of the world... What would happen if someone were to use that power on their own body?
This practice is known as Hubris, and its mortality rate is extremely high due to the sheer difficulty of precisely modifying one's vital functions without killing themselves.
This is Paige's main call to fame, the ability to perform Hubris at will on her own body whenever she switches Memento, for the reasons detailed in her character page.
Some NPCs perform (or try to) Hubris at different points throughout the game, mostly as a last resort to stop Paige in the Bloodbath Route (Tsubame just enjoys it I guess).