Based on the song NEGAI from the rhythm game NOSTALGIA.

“I’m off!”
When I told her about the God who gave Okuu her powers and the humans who trespassed in our house, Koishi ran out of the house with the force to jump out. It’s good that she’s fine, but I wish she’d at least tell me where she’s going. But I know what to expect.
The only thing I can’t read is my sister Koishi’s mind. That’s because Koishi closed her third eye, something I have as well, a long time ago. That made her unconscious and hard to perceive, so even I can’t see her sometimes. As for her, she seemed to be happy with the result, but I still felt responsible for it.

“You’re too kind, big sis. I can’t.”
She said sadly and laughed as if she had cried. Shortly thereafter, on a rainy night, she was gone. I didn’t know if I just couldn’t perceive her or if she was really gone, so I mobilized all my pets to look for her. We went to the surface, which we didn’t want to do, but it was useless, and about a month later, she came back, despite our worries.
“Please, please tell me if you’re going to go somewhere, I’ll be there. When you come back, I’ll be here, too. We may never know if you’re gone again, or even if you are dead.”
As I hugged Koishi and desperately tried to tell her what I was thinking, tears began to flow down my face. I’m so close to her, but I’m really far from her heart. She was confused as if she couldn’t understand my sorrow, but she said, “I understand, I’m sorry sis,” as she patted my head.

“I’m home, big sis.”
“Waaa, you surprised me!”
“Tee-hee.”
A voice suddenly came from behind me. Lately, Koishi has been surprising me like this. It’s good that she properly says she’s home, but she usually surprises me, so I hope she stops.
“How was it on the surface?”
Our relationship hadn’t changed before or after Koishi closed her eye. She had always admired me and was a bright girl. She is my precious sister, even though our emotions have been diluted.
“Uhm, you know what?”
Koishi hugs me. Did something sad happen on the surface? Something that would make her sad right now would be of great importance. I immediately ask her what’s wrong.
“Thank you for everything, big sis. I’m sorry I made you worry.”
At that moment, I saw a little bit of Koishi’s heart. It’s a thin, filmy vision, and it’s far from the heart she used to see, but it’s still nostalgic. Your heart has so much color, Koishi. I loved it.
“…Why are you crying, big sis?”
Koishi smiles amicably. Immediately, her heart is concealed. Maybe the third eye wouldn’t close perfectly unless it was crushed or something. The present moment gave me a little hope. I was determined to take care of this dear little sister forever, and I was strong enough to tell her that I wasn’t crying.