Dad's parents are the closest blood relatives Mia has after the accident. They stay by Mia's side, which is bittersweet, because they are family members who love her, but their physical resemblance to her father reminds Mia that her parents are dead and gone.
Gran and Gramps are similar, yet different. Both love and support Mia, but they have different ways of showing it. Gran is more verbal and affectionate. Gramps is the strong, silent type; he prefers handshakes to hugs. The rare embrace from him really means something.
Gramps is also a skeptic. He thinks Mia may be able to choose between life and death, but Gran says, "It doesn't work that like that" (9.8). Boy, is she wrong. Gramps encourages Mia to choose, saying, "It's okay if you want to stop fighting" (14.19). Ironically, Gramps saying that Mia can go encourages her to stay. She knows her grandparents will support her, no matter what she chooses.