She hails from the Adrestian Empire (being from the previous Black Eagles group) and is quite fond of Edelgard.
The real Monica von Ochs is never encountered in-game in Three Houses, as she is killed and impersonated by the assassin Kronya prior to the events of the story.
However, in Fire Emblem Warriors: Three Hopes, she appears in the flesh when she is saved from Kronya by Shez. Monica then becomes a playable character for Edelgard's story, Scarlet Blaze. She possesses a minor Crest of Macuil and is specialized in wind magic as a result.