ive been thinking about this too, but maybe its just that letio grew up around thieves. it is also said that in the novel, the fl (i forgot her name) is the one who taught letio the etiquette, which is now selena's job.
but it could be though, letio has been suspicious lately ┗( T﹏T )┛
About your theory, i think, i just think that the title 'the villain discovered my identity' i think the 'villain' is pertaining to Letio since he did know Selene's real identity from the start even though she was pretending to be Celestine. And the prince still doesn't know she's a girl unlike Letio ╮(￣▽￣)╭
This!
This what..?
Oh sorry I didn't explain more, what I mean is I agree with the theory you wrote above (๑•ㅂ•)و✧
OKAY THEORY
The title is called the villain discovered my identity. But the story villian hasn't found out she is a girl. SO what if the mc of the story. Is the actual villain, because the story was centred around him, so the story was only really saw from his perspective.