Eitri

Eitri (Japanese: エイトリ Eitri) is a playable character appearing in Book V of Fire Emblem Heroes. Eitri is a skilled inventor and a sage dwelling in the forests of Niðavellir. Initially presenting herself as an ally of the Order of Heroes due to their shared enemy in Fáfnir, Eitri is revealed to be the main antagonist of Book V as she assists Ótr in kidnapping the Summoner for her own ends and is responsible for Fáfnir's corruption.

(Source: Fire Emblem Wiki)