Since Junhyung got the most votes, he's going to have the first alternate ending, which will probably go on for five or more chapters :)

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"____-ah!" a familiar voice called. You turned, and couldn't believe what you saw. Lee Joon was back.

"Joon o-oppa..."

"Are you okay? Mir's being a jerk, forget about him. I can't believe I actually left, I'm so sorry..."

"It's okay," said Junhyung behind you. "She's in good hands."

Joon stiffened, and straightened up, "Yong Junhyung."

"Yes, is there a problem?"

"Not really. I--"

"If you're thinking about wooing my sister, I suggest you wipe away all thoughts of that from your head right at this moment. She's not interested," said Junhyung coldly, taking you by the shoulders and steering you away from Joon.

Joon stood there, dumbfounded. Once he regained, composure, he started calling out your name. Your heart broke hearing him cry out, but you had to leave. You just couldn't take it anymore.

You and Junhyung settled down in a deserted classroom, and you were thankful for the silence that engulfed you both. 

"Thank you, oppa," you said, "for everything. We're not blood related and yet you're so nice to me." You laughed a bitter laugh, "I initially thought you'd be mean to me, you know. And you were."

"____-ah--"

"I'm sorry for being such a burden to you, oppa," you said, tears falling from your eyes. 

Junhyung walked over to you, lifting your chin with a finger, "You're not a burden. I love you, ____-ah." And he kissed you on the forehead tenderly. "I've loved you from the very start."