Kronya

"Just the sight of your face makes me so angry! Can I kill you now, just to get it over with? Please? Gah! Why not? It would be easy. You're such a weak creature that I'd hardly have to lift a finger. Fighting alongside you, sharing our victories and defeats... I'm beginning to feel attached to you. I don't know why. I'm an Agarthan, and you're just a pathetic little worm. It's unthinkable. It's repulsive! But that's how I feel. So if you won't let me kill you, you'll have to agree to be my plaything. Forever. All right?"

- Kronya threatening the Summoner in Fire Emblem Heroes

Kronya is a supporting antagonist in Fire Emblem: Three Houses and a member of Those Who Slither in the Dark. During the Acadamy Phase, she disguised as a student named Monica, presumably done after killing the real one, and inflitrated the Garre Mach Monestary, turning other students into Demonic Beasts and killing Jeralt.

She is voiced by Marika Kōno in Japanese version and by Colleen O'Shaughnessey in English version.

Trivia

 * Kronya is the only few villains that does not get an adaptational heroism as a summonable character in Fire Emblem Heroes. She maintains her condescending attitude and even sends death threats towards the Summoner, who earns some sort of respect and friendship from the likes of Zephiel, Gharnef and even the misanthrophic Grima.
 * Ironically, while she became the first member from Those Who Slither in the Dark that became playable in Fire Emblem Heroes, she never has more impact on the entire story (unlike all others from Three Houses) other than killing Jeralt, and is slain one chapter after that.