Rei's grandfather is Rei Hino's grandfather who appears in both the anime and the manga.
Rei's grandfather is good-hearted but a bit fond of girls which exasperates Rei. He takes care of Rei because she did not want to live with her father , who is a prominent politician in the National Diet (parliament based on the Prussian system) of Japan.
In the anime, he is shown as being very goofy. Consequently, he and Usagi get along very well. He energetically encourages temple visitors to buy lucky charms, make wishes, pray, etc. Rei gets very worried about him because he is very jumpy and physical for a man his age. But despite his goofiness, he does manage to come down to Earth when the situation warrants it.
In Episode 30, "Grandpa Goes Crazy, Rei's in Jeopardy", it is found out that he is a keeper of one of the Rainbow Crystals, in which Zoisite attacks him for.
In the Sailor Moon manga, Rei's grandfather does not appear often. He is first seen in Act 5, who comes to tell Rei that Usagi and Ami (and Makoto) are here to see her. He offers them jobs as miko, which causes Rei to become irritated.
He appears again in Act 41, where he has come down with a cold, and Rei has to take care of the shrine. He remembers when Rei came to live at the shrine before starting elementary school. He is the one who told Rei about crows, and was there when Rei ran towards two crows and named them Phobos and Deimos. He notes that Rei has always been a mysterious girl, and that one day, she will leave Hikawa Shrine and seek her own future.
He also appeared in the Casablanca Memory sidestory.
- Naoko Takeuchi was frequently hit on while she herself worked at a shrine. This led to Rei's grandfather being a pervert.
- Bishoujo Senshi Sailor Moon shinsouban Volume 3