Ur rant makes me doubt u understand what reading comprehension is.
Is Rista annoying? Yes.
Am I going to continue reading? Yes.
An author can and will introduce annoying characters into the world of his making, and will direct us to empathise with the main character more than other, rain is simply minding his own business and this girl is annoying him, ofc ill be annoyed when mc IS being annoyed.
Looks more like you don't understand reading comprehension.
Mc can be as annoyed as he wants and she can piss us off as much as she wants. The fact that she's annoying does not remove the aspects of the world and the mc that we wouldn't think about if she didn't call attention to it. Could the author have left her out? Yes, but the world would be that much flatter and while it'd be a less annoying read, rain would be more boring of a character.
Reading this comment section makes me realize just how little reading comprehension yall have.
Is christina or whatever annoying on a surface level? Yes. Is she also a character integral to the world building and even building of Rain's own character? Also yes.
(It's been a while so I don't remember everything off the top of my head but here we go)
Her story shows that there's more to the noble mage families than just a bunch of high ranking families that peoduce strong mages. It shows that there are power dynamics that fluctuate over time between the families which could open up to other plot things later in the story. There's probably more to her as well but I don't remember.
She's also a good character in her own right with her own full agency. She's not just some girl of random standing in the school who can't help being wrapped up in all the MC's antics to eventually become a bland love interest. Honestly, the MC wouldn't even care about her if it wasn't for her drive to be the best and subsequent rivalry, albiet quite one sided, with him. Her drive and ambitions are her own and let's be real, in a different story she is the protagonist.
She also servs as a good foil to the MC, aka a character that emphasizes another's traits by being the opposite of them. They're both a bit stuck up, but his comes from a place of once being in that high position and now superior knowledge while she on the other hand is working her ass off to maintain that position with what she can achieve now. The MC is also a person who lives for the past, most of his story up to this point being about him figuring out what happened after his death. She on the other hand lives for the present. He has space to fuck up because everything he's working for had already happened, however if she messes one thing up it could ruin everything she's put her life into making for herself.
They both have things they can learn from each other, and while yeah I do find her annoying, all the people saying she's pointless and annoying and to just take her out of the story are completely missing the point of her character.
Anyways I'm gonna go sit in the corner and cry cause why did I just write an english essay on a character I don't even like okay bye.