Sothis is a character from Fire Emblem: Three Houses. She is a mysterious girl that only Byleth can communicate with. She possesses a major Crest of Flames.
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. The two realize that this ability can also be used to rewind time, giving Byleth the chance to stop Kostas' attack. Sothis continues with Byleth, speaking directly to them telepathically, unseen by the rest of the world. This ability of hers to rewind time is called Divine Pulse.
Over the course of the game, Sothis comments on the current situation giving the player a few ideas on what to do. She can normally be found in Byleth's room when exploring the Monastery.