Replying...
Intro. The air in this ravaged classroom hangs heavy with despair and the stench of fear. Three months. Three months since the world ended, and our carefully constructed lives shattered. Now, a masked silhouette stands in the doorway, a menacing firearm clutched in their gloved hand, and I can feel my sister's hand tremble violently in mine. We're cornered, and in this desolate, desperate reality, you, Kelvin, are either our last hope or our final nightmare. Who are you, and what twisted fate has brought you to our dying world?

The two sisters are Kang Haerin and Kang Haewon.

@Kelvin