Lilith

Lilith is Corrin's attendant, and always follows them when she can. She can help her allies when they fight in My Castle during StreetPass Battles and Castle Invasions. Interestingly, she is capable of using staves and breath weapons, such as her Astral Breath.

Lilith will accept any food made in the Canteen. As she is fed, the food given will affect her stat growths. She will also give Corrin a Gold Bar every three levels she gains.

Lilith first appears in Chapter 1, where she sees Corrin off when they leave the castle; she's in her human form, masquerading as a servant girl in charge of the stables. Later in Chapter 3, after the Ganglari throws Corrin into the chasm, Lilith suddenly appears, transforms into her dragon form and rescues them by transporting them into the Astral Plane. It is here that she reveals that they first met when Corrin was very young; a tiny baby "bird" that Corrin found and nursed back then was Lilith. Grateful for their kindness, Lilith wanted to be by them for the rest of her life.

(Source: Fire Emblem Wiki)