I Became the Older Sister of the Regretful Male Protagonist with a Limited Lifespan.

I possessed the body of the manhwa character—the male lead’s older sister, who tormented the heroine and piled up all sorts of bad karma.
In the original story, she dies of an incurable disease, which becomes the reason the young male lead turns cold and twisted.
…Yeah, no thanks. I refuse that fate.

Luckily, I already know the cure.
To heal, I just need continuous physical contact with someone overflowing with divine power.

And that someone happens to be Prince Kallun, the heroine’s older brother and the empire’s infamous troublemaker.
So, I decided to aim for a contract engagement with him.

“If our family can be of help to you, Your Highness, please use us as you wish.”
“…Don’t even dream of marrying me for real.”

He’s cold and prickly, sure—but who cares?
Just holding his hand makes my illness disappear. He’s basically my walking cure.

Now that I’ve secured my “medicine,” it’s time to enjoy a luxurious, lazy, golden life.

“How could you be so foolish?!”
“P-Prince Kallun is a good man!”

I play the role of the tragic romantic when needed—

“This wasn’t coincidence, was it, Lady?”
“…Heh, do I really need to answer that?”

—and the scheming villainess when it helps.
Before I knew it, the day of our “breakup” arrived.

But then…

“…It was my fault for using you. What must I do for you to forgive me?”

Wait—why is he repenting and begging for forgiveness?
I was the one who used him as a cure!
Why are you suddenly turning into the regretful male lead?!