The parents passed away while he was still young.
But he had a little sister he held very close.
Sadly to say, it would not remain that way.

And then the time came for them to separate.
The brother lamented not being able to protect his sister as he'd resolved to.
Their final promise and farewell.
"When we become adults, if anything bad happens you can count on your brother! I'll help you no matter what!"
"Alright, promise..."

After 10 years, the brother sinks into thought, wondering how she's doing.

On the way home, he turns a corner and comes across that girl.
Astonished, she looks at the boy, who doesn't quite know how to react.
"Onii-chan...?"

The boy takes his sister back home for the time being, as the rain has began pouring.
Did he worry about her all the time? Was he fine with being apart?
Hearing all of his little sister's worries, he resolves once again to protect her.
And so they step forward. To make memories together, making up for every minute they were apart.

[Translated from the official site]