Sothis (ソティス, Sothis) is a character who debuts in Fire Emblem: Three Houses, as an entity that appears in Byleth's dreams and eventually manifests into a human form. In Super Smash Bros. Ultimate, she appears in Byleth's Final Smash as well as their reveal trailer.
Sothis is the progenitor god of the continent of Fódlan in Fire Emblem: Three Houses. Many years before the start of the game, Sothis arrived in Fódlan and created her children, the Nabateans, as well as gifting the humans who already lived on the continent, the Agarthans, with great power and technology. Eventually, the Agarthans became greedy and arrogant and attempted to wage war against Sothis and the Nabateans, only to nearly destroy themselves and the continent. Sothis expended most of her energy to heal the continent, then fell into a long slumber, while the surviving Agarthans retreated underground.
The Agarthans eventually recruited a bandit named Nemesis to enact revenge on Sothis and the Nabateans. Nemesis and his warriors, the Ten Elites, slaughtered the sleeping Sothis as well as almost all of the Nabateans, using their hearts to grant themselves Crests and their bones to create the Heroes' Relics. The five surviving Nabateans, led by Sothis's daughter Seiros, allied themselves with one of Nemesis's own and eventually destroyed Nemesis and the Ten Elites. Following the war, Seiros rewrote history to hide the truth of the Heroes' Relics and Crests, creating the Church of Seiros. By the time of the game, Seiros has taken on a new identity, serving as the Archbishop Rhea.
Rhea plots to revive Sothis, bestowing Sothis's Crest, the Crest of Flames, upon a woman of the church. The woman, whose name is Sitri, falls in love with and marries Jeralt, the captain of the Knights of Seiros, and becomes pregnant; however, the child is stillborn. Sitri pleads with Rhea to use her Crest to revive her child and Rhea relents, though the baby remains without a heartbeat. Suspecting Rhea is not telling him the full truth of what happened to his wife and child, Jeralt fakes his and the child's death and flees the Monastery. This child grows up to be Byleth, seeing Sothis in their dreams; however, Sothis cannot remember her past. With Sothis's help, Byleth gains the ability to rewind time.
After a chance encounter results in Jeralt and Byleth being brought back to the Monastery, Rhea continues her plan to revive her mother. Sothis slowly regains her memories and eventually realizes that she is the progenitor god, but is forced to merge her power with Byleth in order to allow them to escape a dark realm they were banished to by one of the remaining Agarthans. Although Byleth now has the full power of the goddess, Sothis no longer appears to speak to them. Rhea attempts to have Sothis take over Byleth's body by having them sit on the throne, but Sothis refuses.
It is possible for Byleth to fall in love with Sothis and select her as their S-Support at the end of the game, even in Crimson Flower, despite Byleth's crest being destroyed in the route; should these conditions be met, Sothis will regain her form in Byleth's dreams.
She is also a Legend-rank spirit, whose unique skill, Critical-Health Healing ↑ ↑, restores the user's health by 100% once the player has taken 100% damage or reaches 20% of their Stamina; however, it can only be used once per match.