Spoilers : The Abandonned Empress

Spoilers : Ji-Eun, Tia and Ruve's relationship in second timeline

When Ji-Eun arrives this time, she does everything she can to seduce Ruve because she wants to make him fall in love with her so she can then toss him aside and destroy him, the Empire, and Zena.

But at this point Ruve only loves Tia and none of Ji-Eun's attempts work. So she instead asks Zena to adopt her, so she can have the aristocrat backing and become Empress. Her second goal, of course, is to spy on Zena and find out his weakness so she can kill him.

When Jieun comes, she tries to seduce Ruve to make him fall in love with her then abandon him, like he did to her. But Ruve gives her the cold shoulder (he has absolutely zero interest), and Jieun soon realises that this Ruve is a completely different person. He is not the man she used to know at all. So her revenge on him is pointless.

When Jieun arrives, Tia is convinced that Ruve will fall for Jieun once again and that she will become empress.

When Jieun is to be adopted by a noble to be on equal foot with Tia to compete to become empress (that's a twist of the second lifetime) Tia is even ready to accept Jieun as her older sister and Keirean offers to adopt Jieun. Carsein's father also offers to adopt her. But Jieun turns her back on the imperial faction to join the aristocracy faction instead, becoming the adoptive daughter of Duke Jenna.

We understand later that Jieun wants to spy on the Duke, but this is a move that turns the tables. For it is now impossible for Jieun to become the empress. She's become an ennemy of the emperor and those who support the imperial power. So Tia is the only hope left to the imperial faction and Ruve (although the faction will try to introduce another contestant, Grace, when Tia is poisoned and declared infertile - but Ruve, as I said earlier, will never consider anyone but Tia as his empress).

Jieun tries to seduce Ruve at every turn, so the love triangle is thriving. She also seeks the support of the Temple (the Church in a way) - which she gets. Tia and Jieun don't get along at all, for they have opposite personalities and different sensitivities. Their confrontations always end badly, because Tia doesn't understand Jieun's hatred and anger towards her. Tia just want to live a peaceful life and doesn't understand why Jieun wants Tia to "compete" with her so badly. That's when Jieun explains to Tia what happened after she died : Ruve and everybody were always comparing her to Tia, and everybody regretted Tia.

This angers Tia because she was DEAD... is it supposed to comfort her to know people regretted killing her? She doesn't feel like she won anything, or did anything that justify Jieun to run her to the ground.

Zena though is smart and while he wants to manipulate Ji-Eun and make her Empress, doesn't trust Ji-Eun at all.

By the way, Ji-Eun has a special power where she can heal flowers/plants, just not humans. Tia's power from God is that she can hear Vita's voice/holy messages, just like some priests can.

In the second timeline, Ruve is already madly in love with Tia by the time Jieun comes to their world. He even says to Tia "There is already one woman in my heart, so there is absolutely no place for Jieun or any other woman".

Jieun came a year earlier and emperor Mirkan is still alive. Mirkan definitely shows that he only considers Tia as the possible future empress. But there's no need for that because Ruve is completely besotted with Tia and will not consider any other woman for his empress although Tia keeps on rejecting him.

(He grovels and suffers A LOT, I tell ya.)

Ah another important revelation is about Tia's mum. Duke Jenna reveals that Jeremiah is his niece, the daughter that his little sister had with a simple knight (a commoner). That makes Tia unfit to become the empress, but as it cannot be proved it is irrelevant. Ruve is very sweet about that with Tia and tells her when they're alone : "You mother was amazing" and tells her about his encounters with her mother. Then he says to her quite desperately and strongly : "I don't think noble blood matters, I never did! Don't let this trouble you please."

Maybe Ruve is thinking of his own origins and he thinks this is something he has in common with Tia. That passage in the novel was actually quite sweet. Ruve is a step ahead of Tia's feelings who hadn't realise how this personal matter could distress her with the whole "Jieun and factions things" going on.

In the novel, after Jieun's arrival and adoption by Duke Jenna, a contest starts between the two girls to become empress. But Tia is slowly poisoned with the tea she drinks. We don't know yet who's the culprit. At the beginning of volume 4 she has escaped death. It's the same poison that was used on her mother, and also the same she had in the first timeline - the one that made her barren and had her lose her child.