Grandpa Frank Miller is Junie B.'s grandpa and one of her closest adults, as he can easily relate to her. He is believed to be the maternal grandparent of Junie B. Jones; as seen in Junie B. Jones Is (Almost) a Flower Girl. Grandpa Miller is married to Helen Miller, Junie B.'s nice but serious grandma. Grandpa Miller loves lemon pie and likes to eat it with Junie B. He loves to play games with Junie B. although she wins all the time. Although he is laid-back, he is more serious when Junie B. gets in trouble and gets upset if Junie B. disobeys him or another person. His first appearance is in Junie B. Jones and a Little Monkey Business, when he and his wife get excited about baby Ollie and his wife affectionately calls him "a cute little monkey".
In Junie B. Jones Is Not a Crook, he buys her black faux-fur mittens for "no good reason" and Junie B. adores them. Unfortunately after leaving them behind, they are stolen. After Junie B. loses the fuzzy mittens he bought for her, he talks about how he lost his wallet at the mall but someone then returned it the next day. Frank is relieved because his wallet has a beloved photo of him and Junie B. He inspires Junie B. Jones to return the pen she found that wasn't hers, despite her liking of the pen.
Junie B. also enjoys helping Grandpa with odd jobs at the house. Their favorite project is fixing the upstairs toilet.