Why Is My Cool-Beauty Little Sister's Yandere Best Friend Fixated on Me?

I'm Su Mu, a sophomore in the history department at Jing University, and the eldest young master of the Su family.

By all rights, I should be a prodigy destined for greatness.

But the biggest frustration of my life is having a younger sister, Su Tang, whom the entire upper crust dotes on.

She's cool and aloof yet sweet and soft, like a little cake. The whole family treats her like she's made of glass, and I've spoiled her for nineteen years, too.

But can someone tell me why her best friend, Ruan Qingci, who is so poor she can barely make ends meet and seems so gentle and well-behaved, suddenly started acting so strangely?!

That day, I had just stepped in front of my sister and warned that woman, "Stay away from my sister."

But this lunatic didn't back down. She tilted her head and gave me a strange yet sweet smile:

"Brother Mu, why are you always standing in front of me?"

"Since you're so worried about her, why don't I claim you first?"

Help!

I just wanted to be a doting big brother. Who wants to be targeted by his little sister's crazy, possessive best friend?!

How is this plot even more ridiculous than my own dreams?!