Kasatani Youko

She is Rie's older sister and a 22-year-old female college student.

She takes care of Rie in place of her parents who live abroad for work.

She has a calm personality and is always temperate and kind.
Unlike her sister Rie, she is fond of the main character and treats him like a younger brother.

The main character also admires her and looks up to her like an older sister.

Translated from the the official site