Those spoilers are all from novel updates.
All I did is sumerising them.
Also, don't take me wrong, I'm absolutely not trying to defend Ruve but I still wish to explain why things happened the way they did in this timeline.
I just want to give the rationale for the actions of the first timeline.
So the background is, Ruve's father, Emperor Mirkan, has always been very cold and distant to him, raising him with harsh expectations and no love.
The Emperor was actually in love with Tia's mom, Jeremiah.
Keiran (Tia's dad) was engaged to be married with another woman (Karsein's mother actually) in a political marriage, but he also fell in love with Jeremiah.
Jeremiah was in love with Keiran, not the Emperor. The only person in the empire who can break off the official engagement between Keiran & Karsein's mom is the emperor, but obviously the emperor wants them to get married so he can hopefully marry Jeremiah.
(Jeremiah died poisoned by Duke Jenna. He already tried to have her killed by assassins but she was rescued by Keirean and the King. They both fell in love with her on that day.)
Jeremiah is the daughter of Duke Jenna's young sister and a knight she eloped with. Keirean was engaged with Mirkan's sister (Carsein's mum) and he was unwilling to swear the Monique oath to the emperor.
The Monique oath is a way for the emperor to bind the Monique family's loyalty to the crown and make sure they stay servants to the crown. Because some members of the Monique family sometimes ruled the empire in the past, each new emperor waits for the heir of each generation to swear the oath in exchange for a wish granted.
Keiran offers to swear an oath of absolute loyalty to the Emperor if he will give up on Jeremiah and allow Keiran to marry Jeremiah by breaking the official engagement.
The Emperor/King is actually a very just, wise man and he realizes he cannot put his own personal feelings ahead of the good of the Empire, so he gives up on his love for Jeremiah and breaks off Keiran's engagement so Keiran can marry Jeremiah and then Jeremiah gives birth to Tia.
Just some background, I'm probably mixing them up (?) but Tia's family is part of the "Aristocrats" (I think) which mainly support the Emperor and on the other side you have the "nobles" or something, which oppose the emperor, the faction which Duke Jenna is part of.
The emperor is actually not as powerful as you think, as the noble faction have basically 50% the power of the empire and the Emperor and aristocrats the other 50%. The emperor needs to ensure the aristocrat faction that supports him is unified (there's in-fighting inside both factions), which is why he wanted Keiran to swear an oath of absolute loyalty to him.
Tia has her father's hair coloring, but looks exactly like Jeremiah, which is why the Emperor (who was in love with Jeremiah), loves her so much and treats her like a daughter. The reason behind that was that emperor Mirkan was in love with Jeremiah, Tia's mother, and considered Tia like his daughter.
But one day, when Jeremiah and Tia were visiting the palace, assassins broke in.
And when they attacked, Jeremiah unconsciously protected Tia with her body, leaving Ruve defenseless. It crushed Ruve's heart to see that Jeremiah didn't consider him like a son. All of this fuelled Ruve's spite against Tia who had everybody's love.
Only two people ever showed "love" to Ruve, which is Jeremiah and Duke Jenna/Zena (or whatever his name is, the main villain).
However, when Jeremiah gets assassinated later on by him, she "happily" gives up her life to protect Tia, while ignoring Ruve (who was also in the same room), so Ruve feels betrayed that Jeremiah didn't care about him at all, which I mean, of course this woman will protect her biological daughter over the unrelated prince.