Someone posted is at the bottom, but basically Jieun had everyone turn on her after Tia died. So she has the chance to go back in time and she does it because she wants to get revenge on Tia (who she feels has wronged her), she wants to get revenge on Ruve and the other aristocrats including Zena.
Eventually she and Tia work together so they can bring down Zena and he ends up dying. Everyone goes after June though, so when she is in jail she and Tia talk with each other and they clear up all the misunderstandings. Tia let’s Jieun get the opportunity to leave by faking her death and she goes to another country.
I like Jieun after this because she was able to reflect on her actions and realize that she was in the wrong for blaming Tia, but also because she went back in time to get revenge on Zena. I like that afterwards she didn’t cause any problems for Tia and decided to leave and live her life. When you think about it she was just a high school student who got isekai’d so I don’t blame her too much for being clueless, she got with Ruve because she was scared of dying.
(There are probably things she will do i between that time that will make me hate her, but for now I am happy knowing that she became a better person and not an idiot like before.)

After reading the spoilers I think Jieun is probably my favorite character.