I have to agree with you on this point. The three main characters are just there. Without the inmates coming back there would be no reason to focus on them. There should be a reason and development as to why the main characters are important and we should care about them rather than just being tied to the weekly event of an inmate returning.
They setup some of her background in the pilot regarding her father and grandfather, now they just need to develop it more.
My favorite character is the warden. The actor is really good and there is so much mystery, creepiness, with the character that I can't get enough of him.
It's a good show but it isn't memorable just yet. I think it can be though because the cast is very good and the concept is also intriguing.