Mary knows that she lives in a weird painting world and that outside of this world, there is another one where she might find her "father."
However she is not aware of the fact that
she is not a real person and a painting. Specifically Guertena's final work.
When she reads the art book, something messes with
making it unreadable.
Similarly, that entry gets swapped with the one for "Marvelous Night," if Ib, Garry, and Mary read it when they are together.
However when Garry reads the entry and find out that she is not human, the "Strained Ear" hears Garry read her entry, and the "Tattletale" then "tells" Mary the secret... However she momentarily goes crazy, but returns to her old self soon afterwards, without the knowledge of what she had just heard.
In one of the endings...
"A Painting's Demise," where she leave Ib and Garry behind and just tries to leave the Fabricated World... something stops her.
Is there some unknown third pulling the strings? If so, who is it and why are they doing this?