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.

(Source: Girls' Last Tour Wiki)