Her goal is to help Byleth out by saving Fódian and possesses a major Crest of Flames. Near the end of Fire Emblem: Three Houses, she fuses with Byleth to defeat their enemies.
She is voiced by Tomoyo Kurosawa in the Japanese version, and by Cassandra Lee Morris in the English version, the former of the latter who also voices Morgana in Persona 5, Rachel in Glitter Force Doki Doki, and Suguha Kiriygaya/Leafa in Sword Art Online.
Sothis is a young girl with beautiful, long, green hair and eyes, wears red earrings, and wears a purple dress with gold details. She also yields a sword on her hand, and wears golden feetless shoes with a purple jewel on each side.
Sothis is very caring and supportive to Byleth, and the rest of her family. She is also courageous and saves Fódian with Byleth. Sothis is also charming and can also fuse with Byleth near the end of Fire Emblem: Three Houses.