Is this Miss Not-So Sidekick...?
Hyejung loved to read to escape her daily stress. But that’s before she woke up inside the bizarre world of her favorite novel! Instead of the main heroine who courts three eligible men, she is now Latte Ectrie – a minor villain that everyone hates?! One way or another, it’s a chance to live out her most beloved storyline, with popcorn in hand to watch all the drama! Taking charge of the narrative takes on a whole new meaning!
A Korean teacher wakes up in the world of a web novel she'd been reading as Latte Ectrie, a blonde noble girl and supporting character.
The heroine of the novel is Ibelin Dott, a black-haired noble girl who, in the original storyline, is romantically pursued by the three most handsome men in the empire: the Crown Prince Rondemio de Helone, Duke Kenneth von Esbande, and Arwin Hebrim, the silver-haired Master of the Tower of Magic.
The protagonist alters the storyline such that Arwin in particular becomes more interested in her than Ibelin, causing Ibelin to feel pangs of jealousy for the first time in her life and souring her friendship with Latte.
It's subsequently explained that Ibelin had never felt a negative emotion toward anyone before, due to having grown up surrounded by nothing but love. However, her coddled upbringing prevented her from becoming emotionally mature, meaning that she essentially thought the world revolved around her and couldn't handle it when she wasn't the centre of attention.
TV Tropes describes her personality thusly:
False Friend: Even before its revealed how much she has come to resent Latte, there are many hints she's not a very considerate person. She seems to only view men as a means for resources, and women as pitiful creatures she has to protect. Despite 'being kind to everyone' she is unable to foster relationships on her own, and expects everyone to initiate so she doesn't have to. There are many instances where she is superficially 'worried' for Latte's safety, but doesn't inquire how she's doing after the fact and seems uninterested if she's told. This is the first hint to Latte that Evelyn isn't who she seems.