I feel that the author making Narae like this is just because he doesn't have any better idea to explain why she is not Sungwoon's girlfriend by now.
Let's be honest. Mc and Narae should have been a couple since ... chapter 1? But the author clearly wants to avoid any stable relationships at this stage, and keep the "uncertain" harem mechanics going on.
And so, his way to do this is making Narae into a violent tsundere in denial. Because that way he can keep things paused, and doesn't have to deal with a couple.
Frankly, that's the worst part of this manwha imo.