Cornelia Arnim was originally an Imperial scholar invited into the Kingdom by King Lambert, who was later appointed the high position of court mage of the Holy Kingdom of Faerghus in recognition of her deeds in saving the people of the Holy Kingdom of Faerghus from an epidemic. She is first mentioned by Sylvain during Part I.
On the Crimson Flower route, Cornelia alongside Rodrigue guard Fort Arianrhod when they get the news the Edelgard and the Adrestian Army is about to lay siege on them. Cornelia at first shocked upon hearing this, as she had not been given any prior intel of the Empire's sudden change of plans, but later calmly accepts this situation and choses to prepare for the upcoming fight by unleashing her Titanus on the invading army, secretly hoping some of the Kingdom forces would end up killed by them in the ensuing chaos. Once she's finally defeated, Cornelia takes her death with grace and simply laughs as she realizes everything that has happened has gone according to the Empire's plans, calling it a "delightful dance". Once Edelgard and her army arrive back to Garreg Mach, Arundel suddently shows up and confronts her for her rash decision in concealing their true target from her allies, as Cornelia used to work for him and could've been potentially spared and even taken down Arianrhod in their stead had she been alerted before.
During Part II of the Azure Moon, Verdant Wind and Silver Snow routes, Cornelia frames up Dimitri for the murder of his uncle Rufus, the Kingdom's regent, and has him apparently executed. With him gone, Cornelia pledges her loyalty to the Empire and turns the Holy Kingdom of Farghus into the Farghus Dukedom.
On the Azure Moon route, the Dimitri and Kingdom Army eventually marches towards Fhirdiad, which causes the oppressed citizens in her rule to revolt against her tyrany. Unable to divert forces to suppress the rebellion in the face of Dimitri's army, Cornelia grumbles about how he and Edelgard should have just killed each other and deploys Titanus as well some Viksams to face his forces. Once Cornelia is defeated and mortally wounded and, right before perishing, Dimitri's demands her to reveal the truth about the Tragedy of Duscur, as he suspects she may have been involved in it. Upon hearing Dimitri's request, Cornelia claims Dimitri's stepmother Patricia had a key role in the Tragedy of Duscur and confesses to helping her orchestrate the massacre in excange of the promise of seeing her daughter again.
On the Verdant Wind route, NPC dialogue mentions Cornelia suddently vanished from the Farghus Dukedom right after Edelgard's death, leaving her whereabouts unknown. The same applies to Silver Snow as both routes share a similar storyline.
It is mentioned Cornelia had sudden shift in personality sometime after being granted her new position in the Kingdom, and considering her connections to "those who slither in the dark" and her access to their resources is consistent with other characters killed and replaced by Agarthan infiltrators, this suggests she was murdered sometime after solving the Kingdom's epidemy and before the Tragedy of Duscur, and the woman the player has known as Cornelia is actually a high-ranking Agarthan impersonating her.
Role
Cornelia is a non-playable character on the Silver Snow and Verdant Wind routes, and the boss of Chapter 16 on the Crimson Flower route and Chapter 18 of the Azure Moon route.
While presenting a front of an affable servant of the Kingdom, Cornelia is in reality a cruel woman who's willing to do whatever it takes to ensure her dominance of the Kingdom should the situation present itself, views the Dukedom Forces as nothing but pawns and has no care for the people living in it. In contrast, it is mentioned by multiple NPCs as well by Dimitri she used to be more kind in the past, and that she even had different likes and dislikes compared to the woman seen in the story.