Kronya

Kronya is an antagonist from Fire Emblem: Three Houses and Fire Emblem Warriors: Three Hopes. She hails from Agartha and is one of Those Who Slither in the Dark. Kronya infiltrates Garreg Mach Monastery's Officers Academy by posing as Monica von Ochs, an about-to-graduate Black Eagles student who had gone missing a year prior. During the Horsebow Moon of 1180, the Death Knight kidnaps Flayn; when Byleth and their class rescue her, they find an unconscious Monica at her side. Monica quickly recovers and re-enrolls in the Officers Academy as a member of the Black Eagles. Other students and members of the Monastery observe that she's grown quite close to Edelgard and her personality has completely changed. Her personality shift is largely attributed to her apparent year in captivity, though in reality, it's simply Kronya's true personality peeking through her disguise. Kronya reveals her true identity after Those Who Slither in the Dark ……