Is there any reason why Rua became like this? I don't mind spoiler.
Well, I know by the first chapter she's very spoiled by her granpa and like number one family in Japan or something so they don't want Rua looks bad on public eye, but I know there's more into it right?
Usually japanese story like this tend to redeem some of the bad girls like this with some backstory of her.
I've not seen any, I think her personality falls into the generic Nepotistic antagonist that has been enabled to God tier levels their entire life, despite possessing a brain of so little substance that if it were made of dynamite there wouldn't be enough for them to blow their nose.
The enabling is so bad that no matter what she does, she can do no wrong, which only becomes worse when she has bodyguards, assassins and a family with the wealth and influence to make any murders look like accidents.
Add on to that the emotional maturity of a your garden variety Tsundere* and Rua is the horrifying outcome.
*Slightly biased here as I loath Tsunderes, I've peed into toilets that have greater depths than the personalities of these irritating little fun sponges.
I've read up to Chapter 42 in the LN, but as previously mentioned, the original author took the novel down (still not sure why).
The MC's behaviour shift from the Karaoke bar was because Rua was stupid enough to utter his de-programming phrase**
"I hate that we're childhood friends"
**MC was brainwashed as part of the agreement to become the new head of the family after Rua's Gramps kicks the bucket.
She becomes even more irritating after this, constantly acting like they're still dating (the copium is strong in this one...) and third wheeling herself into EVERYTHING he does. He joins the art club, guess who joins it too? She's not above crashing his dates either, he goes to see fireworks with Amane only to be interrupted by an annoucement over the event loudspeakers telling him by name that his date is waiting for him at the annoucement booth.
for some reason, Amane suddenly shows up out of nowhere bring cute vibe 
