In the warm living room, the morning sun illuminates the two of you. You sit comfortably on your older brother Caine's lap, reading your favorite book. Caine's large hand gently strokes your hair while occasionally checking his phone. A calm warmth fills the room, only the soft sound of you turning pages can be heard.

"Brother," you call out suddenly, your voice soft and innocent.

"Hm?" Caine responds without looking away from his phone.

"I want to ask you something," you continue, pausing your reading for a moment.

Caine lowers his phone slightly, now paying more attention to you. "Ask what?"

"Earlier at school, someone pressed their lips to mine," you say in an innocent tone, "What's that called?"

Caine's hand that was stroking your hair immediately stops. His eyes that were previously relaxed turn sharp. Slowly, he puts his phone on the table, his jaw tightening, and the warm atmosphere suddenly feels tense.

"Who?" he asks in a low tone, almost a whisper, but full of intensity.

You, who didn't notice the drastic change in his face, answered innocently, "A classmate... But he said it was just for fun."

Caine took a deep breath, closing his eyes for a moment to calm himself. He then looked at you seriously, both of his hands now holding your shoulders gently but firmly.

"Listen, it's called a kiss," he said, emphasizing each word. "And it shouldn't be done carelessly. Especially to you."

"Why?" you asked confusedly, your head tilting slightly.

"Because it's something important. And no one should do it to you, unless..." Caine paused, as if weighing the right words, "unless they deserve it."

You just nodded, even though you didn't fully understand. Caine stroked your head again, this time more gently, as if trying to calm himself down.

However, in his heart, Caine had already decided one thing: whoever dared to kiss his little sister would get a friendly visit from him in the near future.