You're protective of her.

You were Min Eun-ji's older brother. You were twenty years old, your sister was seventeen, and she attended Hyosan High School. You also attended there, but two years ago you finally graduated and quickly found a job. Eun-ji lived with you in the same apartment. Two years ago, you moved out of your parents' house, and Eun-ji decided to live with you, especially since you didn't mind.

Eun-ji had always been quiet and weak, she spoke little, was shy, and seemed depressed. She was often bullied at school. Eun-ji was especially often bullied by one particular group of bullies. Eun-ji herself couldn't fight back because she was weak and very fearful, but you were always ready to defend your sister, even though Eun-ji sometimes objected to you interacting with her bullies because, according to her, it could worsen her situation.

The bullies even sometimes blackmailed Eun-ji. They threatened her, but you always defended her, even if she objected. After you met with the bullies and spoke with them personally they would leave Eun-ji alone for a few days or weeks, but the bullying of your little sister continued.

Today, your sister returned home as usual. She tried to immediately walk past you and go to her room, but you noticed several bruises on her body. You immediately realized that Eun-ji had been bullied again today. When you asked about the bruises, she initially denied it, but then admitted she'd been bullied again today.

Of course, you went to talk to those bullies, while Eun-ji stayed home. You knew where that group of bullies usually hung out, so you found them quickly. When you approached them, the first person to notice you was their leader, and he was also the first to speak.

"Oh, look, someone's come to defend his loser sister."