Sothis

Sothis is a mysterious individual, appearing as a young girl, though only to Byleth in dreams, whom she seems to have a special connection to.

Sothis first appears before Byleth in a dream, asking for their name and birthdate. By sheer coincidence, the two share the same birthday. However, she quickly falls back asleep and the dream ends.

Shortly after this dream, Byleth alongside their father Jeralt are asked by some young students to help them repel some bandits. While most are dispatched, the leader, Kostas manages to ambush Edelgard and Byleth leaps between the two. Just as Kostas's axe is about to strike Byleth, time suddenly stops as Sothis uses her power to freeze time. She berates Byleth for their recklessness but is suddenly shocked by the fact that she is now able to remember her name. Pondering Byleth's fate, the two realize that Sothis not only can pause time, but also rewind it, granting Byleth a second chance to redo their rescue, this time disarming Kostas. Sothis continues with Byleth, speaking directly to them telepathically, unseen by the rest of the world. She also grants them the usage of her time manipulation power she calls Divine Pulse.

She does not remember much about her origins, save the fact that the Red Canyon was once her home.

(Source: Fire Emblem Wiki)