Eponine

エポニーヌ, Nina,Wicked Fantasy (邪な空想),Eye Spy

Oh, what are THEY muttering about? Awfully chummy, those two. The fellow my father is talking to... not bad...not bad... I guess there IS a silver lining in being stuck in my father's brigade. Are there any more fine gents worth my attention?
— Nina, daydreaming.

Nina (Japanese: エポニーヌ Eponine) is the daughter of Niles, and an outlaw who grew up in a deeprealm.

Nina can only be recruited if Niles marries a female, and will not be available if he marries a male Corrin.

(Source: Fire Emblem Wiki)