It looks to me that the author is doing what happened to the FL leads personality to this one as well. Flipping from a goodie two shoes to an asshole in the FL's case, and what seems to be the opposite here. We are likely in for a lot of jokes where it's just the MC comparing the FL from the OG webtoon to this new side chick.
I've read the new chapter and started to come up with a new prediction, is the author going to pull A Certain Magical Index Volume 10 type of development?
Tbh not surprised, that was my first guess when she was introduced as the main girl opposite, he main girl is "nice" but the true her is the opposite, so when they said her rival is bad which is opposite of the "nice" fmc, I know the true her probably a nice or shy girl.