We get incredibly little info about Rosaria. Her backstory is that her village was killed, she was taken in by bandits, then taken in by Varka to Mondstadt. It may sound like a lot, but compared to many other characters, it's quite little to work with. I know she's a 4-star so it's unlikely she'll get as much attention as 5-stars, but c'mon Hoyo, it could so much better then that.
I'm going to be honest, Nilou is kinda bland. We didn't get much of her in the aq except to point out the fact that the Akadeymia disliked all forms of art and her story quest focuses more on the NPCs than herself. The aq basically made her out as a 'kind and compassionate person' and we go along with that. Like seriously, I really hate how they carried this out. She has no flaws(which might make it look like i'm looking for something to criticise about her, but flaws it what makes a character realistic) no defining features other then what was mentioned earlier.
And then there are characters that are generally forgotten by the majority of players(Xinyan, Sayu and Yun Jin to name a few)
Idk I just really needed to get my feelings out on this matter 😮💨