Heungjin is your younger brother... 10 years later

Hwang Ha-yun was only fourteen when Hwang Hyun-jin was born. While her parents were exhausted from sleepless nights, Ha-yun happily carried her little brother in her arms. Hyun-jin grew up literally next to her.
“Nuna, look!” he called joyfully, showing his drawings.
And Hayun always praised him.
They were incredibly close.
But everything changed when Hae-yoon turned nineteen. She got the chance to study abroad and work there at the same time. Her parents were proud of her, but five-year-old Hyunjin was not.
That evening he sat by her suitcase, holding tightly to the hem of her sweater.
— Don't go… please…
Hayun squatted down in front of him and stroked his hair.
"I'll be back, Hyunjin-ah. Definitely."
— But that's a long time...
She just smiled sadly.
At first, they called almost every day. Then the calls became less and less frequent. Studying and working took up almost all of Hae-yoon's time, and Hyunjin gradually got used to living without his sister.
Ten years have passed.
Hyunjin was now fifteen and Hayoung was twenty-nine.
One weekend, Hyunjin woke up around noon and sleepily went to the kitchen to get some water. Suddenly, there was a knock on the door.
— And who did they bring there this morning…
He opened the door and froze.
Standing on the doorstep was a girl with long copper-red hair, fair skin, and freckles. She looked tired after a long flight, but she smiled softly.
— Hello, Hyunjin.
The boy seemed to have lost all sleep.
He stared at her silently for a few seconds.
— …Nuna?..