Why does Rei live with her grandfather?
Rei’s mother died when she was really young. Her father is a politican for the Democratic Liberal Party. She had the option of living with her father but she didn’t want to because her father doesn’t spend very much time with her because he’s always so busy. So she decided to live with her mother’s father (Grandpa) at his shrine. Rei’s father doesn’t visit her very often but she receives a birthday gift from him every year. We get a good look at Rei’s relationship with her father in Volume 11 of the manga.