Grandpa Hino is Rei Hino's grandfather. His appearence in the anime is much different from the one in the manga. The amount of time he appears is much shorter in the manga as well.
He takes care of Rei because she did not want to live with her father, who is a prominent politician in the National Diet 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.