The Face-Con Villainess Became the Grand Duke’s Crazy Fangirl

“I’ll make His Highness the Grand Duke fall for me, no matter what it takes!”
I fell for the male protagonist’s younger brother, the Grand Duke, at first sight. My mouth opened on its own, and words spilled out recklessly.
“Please, go out with me!”
*
For three months, I tried every means and method to win the Grand Duke’s heart, but nothing worked on him.
…It was truly time to give up.
“You don’t need to worry, Your Highness.”
“…What are you talking about?”
“I know. I’ve been a nuisance, troubling you all this time. But… that won’t happen anymore.”
“What the hell does that mean, my lady?”
“I’ve given up on you, Your Highness. I don’t like you anymore.”
In that moment, a crack appeared for the first time on the Grand Duke’s ever-composed face. His expression twisted in an instant.
**
As per my promise to my father, I was out to meet a potential suitor, but the other party never showed up. Just as I was wondering if I should head back, a chilling voice came from above me.
“I didn’t know you were such a fickle person, my lady. You said you liked me, yet here you are at a place like this. Was it all a lie?”
It was the Grand Duke.
Hah, fickle?
“I tend to fall out of love quickly, you know. Your Highness should be well aware of that.”
“…Is that so?”
“And it seems you think I’ve only been like this with you. You don’t know anything, do you? I’m a fickle person, as you said, so even when you’re not around, I’ve been with plenty…”
“Why stop? This is interesting. Keep talking. Who else have you been like this with, besides me?”
The Grand Duke’s eyes, as he spoke, seemed unhinged, as if he were ready to hunt down and kill whoever I named.