Sothis is (I think) the first character we see in Fire Emblem: Three Houses. We don’t know anything about her at first. But I liked her story/character overall.
She’s a glowing green being on a throne in what seems to be a dream.
But she “finds” the hero of the story Byleth in the darkness.
And while she doesn’t possess a mortal body, her spirit is there to lend Byleth some support during things in the story.
I think the game says she’s technically responsible for you being able to reverse turns incase you get stuck on a long mission and crucially lose someone you didn’t mean to. Which about saved me countless times during the last two missions.
About halfway through the game, we learn she is a goddess. And not any goddess, the same goddess that the local religion is based solely on. Though her divine power isn’t absolute, as she herself had lost memory of who she is and she doesn’t seem to have much power per se.
Eventually she fuses her soul with Byleth to get the professor out of a dimensional void.
Which gives Byleth a power-boost (though not really to the player) and some nifty green hair.
But after the Byleth-Sothis fusion, Byleth never really sees Sothis again in person. At least during my route. Though the game apparently gave me the option to “marry” her at the end. I opted for Edelgard instead.
I also learned that Rhea was Sothis’ daughter and that she was really a demi-goddess named Seiros. And I learned later (not through my route during the game) that Rhea created clones, and one managed to survive, fall in love with Jeralt, and the clone got pregnant. And the baby survived and grew up to be Byleth. Even more strange, Rhea is an option (depending on your route) for marriage for Byleth.