The hints about Mimi's true identity are literally the vagueposter meme.
#ImperialPrincess maybe? Though, whether she is, but doesn't know it is kinda the question at this point. That said, I'm gonna bust a gut if I turn out right and she's secretly like an Imperial Princess or something similar. XD That or some super high ranking intelligence officer. But, my guess is she's either 1. Princess or other high ranking person secretly "hiding out", or 2. (and this one I suspect more) is that she's an imperial princess or something equivalent, but doesn't know it because her parents went and decided to slum it because one of her parents married "below her station", but didn't tell anyone. Though, if she was actually an Imperial Princess in disguise [and not like a lost relative], then it's SUUUUPER funny in that "Oh, SHIT! THIS IS
SO NOT GONNA END WELL FOR MC WHEN THE EMPEROR FINDS OUT! -dies-" Also, is this a flag for MC being married to other girl too given #YouMARRIEDAWHAT?!APRINCESS!NOFAIR!I'MCOMINGTOO!!!