I did enjoy Monica as a bad person in the game, but despite her beauty, there wasn't much to like. We first see her "rescued" after a year of being gone without a trace.
She seems to have taken a shining to Edelgard and the two are almost BFFs.
Then we find out she's a pawn of the Flame Emperor, who is really Edelgard.
For reasons... she takes Jeralt out with a dagger in front of Byleth.
Turns out she's some kind of clownish demon named Kronya. Apparently the real Monica wasn't her at all. So I'm guessing the real Monica had been murdered.
She offers a decent challenge during a mission.
But I was hoping Byleth would kill Kronya.
But some dark wizard guy sacrifices her at the last moment for a greater source of magic. Good riddance at least.