Grandfather (おじいさん) was the caretaker of Yuuri and Chito when they were young. The two girls have no recollection of their parents, and it is presumed that the grandfather had raised them for a long time.
The grandfather is never explicitly given a name in either the anime or the manga. He is never seen in the present, and only appears in the girls' dreams and flashbacks and has potentially passed away following Chito and Yuuri's departure from their home.
The grandfather appeared to be a well-educated man, with a large collection of books ranging from pre-war folk tales (such as a book about kappas) to historic records of war. He is literate and is presumed to have taught Chito how to read the language used in their universe. It is not specified as to why Yuuri never learned to read from him.
Grandfather also knew how to drive, and mentions how he had taught Chito to drive the Kettenkrad. Yuuri also knows how to drive the vehicle (although not very well), but it is not clear whether the grandfather had taught her also, or she had learned while traveling with Chito.