She's not depressed... she's just numb. A numb person is highly stoic and difficult as a character... because she herself has no idea what she's feeling. That person doesn't even know if she hates the emperor. It's a sort of employed defense mechanism.
To me she does have a depressed personality, her depression being a symptom of her PTSD, or probably, more specifically, C-PTSD (the kind where it's years of abuse and a bit different from normal PTSD).
In the past couple of chapters, she's been trying to get better, and we're getting more of her personality cause she's finally dissociating less and doesn't constantly have to be on guard. And I think we'll start seeing her come out of her shell finally.
I'm not surprised you think she lacks personality tho, because prolonged abuse will change a person into something a lot of people really aren't familiar with outside of the war veteran version.
Again, I still don't believe she has a personality. She's still just a puppet to me and this is mainly because she moves and acts and speaks when needed for the plot and for the characters around her, but we never (or very rarely) get to see her thoughts.
The main dialogue with books and mangas is the MC's thoughts, they're the narrators of the plot. With this story the MC moves and speaks when needed but she rarely thinks or narrates the story.
The main amount of narration and thoughts come from either the emperor or the silver haired prince. Even the maids get more 'thinking dialogue' than the MC.
My point has really nothing to do with her personality or being depressed, it's mainly a flaw with the narration, the POV, a d the author.
Okay so I know this is going to be hated, because how dare there be such a thing as constructive criticism, however I really don't feel that the MC is a character. The MC has no personality, she's literally just a puppet for the author to move around and tell the story of the people around her.
Even when horrible things are happening to the MC she just listlessly moves on. There's no personality. I know she's depressed, but she doesn't even have a depressed personality, she's just there moving about for the sake of other characters.