For Tessa I agree 100%, she has done far too much damage around her to be forgiven and to befriend our team.
But nothing prevents her from changing and becoming a better person once she has suffered the consequences of her actions. (but as far away from us as possible ^^ )
Discovering her past can help to empathize with her, but it doesn't erase or excuse her actions and even less making better the lifes that's she ruined like Aya or the demon's daughters. it's just helps to understand the beginning of the why. (and perhaps she will make an important action to help us later in the story).
Personally, the more the story progresses, the harder it is for me to see her as the final antagonist for Yuki, and given that she seems to be having problems with her father too, I could see him leaving Tessa to deal with it alone, so that he could save his own image, abandoning and replacing her by someone else in the process when all the shit she's done throughout the story comes to bite her in the ass .
Unrelated to Tessa, but I just pray there's still hope for Keiko to be a part of the harem though.