I'm about 7 chapters into the game (BE run), and so far I'm very impressed by the way Sothis is written. She was the character that worried me the most because she ran the risk of falling into very annoying tropes. I was especially worried that she'd just be there for exposition dumps. But at least thus far I find her very intriguing. The best thing about her is that she does not know what's going on. The fact that she does not know why she is in your head and why certain things are the way they are creates a very interesting dynamic between she and Byleth. They're both in this ride together and are learning things together.
In a way, I feel she is what Azura was trying to be. The problem with Azura was that she knew everything that was going on but refused to tell Corrin for no reason. She became very frustrating for this reason and there was no relationship development between she and Corrin.
